CHAPTER XVI
THE GERM-LAYERS AND EVOLUTION
In his papers of 1866 and 1867 Kowalevsky had remarked upon the widespread occurrence of a certain type or fundamental plan of early embryonic development, characterised by the formation, through invagination, of a two-layered sac, whose cavity became the alimentary canal. This developmental archetype was manifested in, for instance, Sagitta,[424] Rana,[425] Lymnæa,[426] Astacus,[427] Phoronis,[428] Asterias,[429] Ascidia,[428] the Ctenophora,[428] and Amphioxus.[428] He noticed also that the invagination-opening often became the definitive anus. Further instances of this mode of development were later observed by Metschnikoff[430] and by Kowalevsky[431] himself, but it was left to Haeckel to generalise these observations and build up from them his famous Gastræa theory. This was first enunciated in his monograph of the calcareous sponges,[432] and worked out in detail in a series of papers published in 1874-76.[433]
Haeckel maintained that the "gastrula" stage occurred in the development of all Metazoa, and that it was typically formed, by invagination, from a hollow sphere of cells or "blastula." This typical formation might be masked by cenogenetic modifications caused chiefly by the presence of yolk. The gastrula stage was the palingenetic repetition of the ancestral form of all Metazoa, the Gastræa.
From the Gastræa theory there followed at once two consequences, (1) that ectoderm and endoderm, invagination-cavity (Urdarm) and gastrula-mouth (Urmund or Protostoma), were, with all their derivatives, homologous, because homogenous, throughout the Metazoa, and (2) that the descent of the Metazoa had been monophyletic, since all were derived from the ancestral Gastræa. Huxley's suggestion (supra, p. 208) that the outer and inner layers in Cœlentera were homologous with the ectoderm and endoderm of the germ was thus fully confirmed and greatly extended.
The great importance of the Gastræa theory lay in the fact that it linked up, by means of the biogenetic law, the germ-layer theory with the doctrine of evolution. It supplied an evolutionary interpretation of the earliest and most important of embryogenetic events, the process of layer-formation. Upon the Gastræa theory or its implications were founded most of the phylogenetic speculations which subsequently appeared.
Upon the Gastræa theory Haeckel based a system of phylogenetic classification which was intended to replace Cuvier's and von Baer's doctrine of Types. This took the form of a monophyletic ancestral tree. Its main outlines are given on p. 290 in graphic form, combined and modified from the table on p. 53 of the 1874 paper and the genealogical tree given in the Kalkschwämme.[434]
Monophyletic Genealogical Tree of the Animal Kingdom, based upon the Gastræa Theory and the Homology of the Germ Layers.
The scheme is in many respects an interesting and important one. The great contrast between the Protozoa, or animals with neither gut nor germ-layers, and the Metazoa, which possess both structures, is for the first time clearly brought out. The derivation of all the Metazoa from a single ancestral form, the Gastræa, leads to the conclusion that the types are not distinct from one another as Cuvier and von Baer supposed, but agree in the one essential point, in the possession of an archenteron (Lankester, 1875), and an ectoderm and endoderm which are homologous throughout all the Metazoan phyla. Finally, in the separation of the sponges, Cœlenterata and Acœlomi as animals lacking a body cavity or cœlom[435] from the four higher phyla, which are essentially Cœlomati, there is contained the germ of a conception which later became of importance.
Somewhat similar views as to the importance of the germ-layer theory for the phylogenetic classification of animals were published by Sir E. Ray Lankester in 1873.[436] He distinguished three grades of animals—the Homoblastica, Diploblastica, and Triploblastica. The first included the Protozoa, the second the Cœlenterata, the third the other five phyla, distinguished by the possession of a third layer, the mesoderm, and a "blood-lymph" cavity enclosed therein. He used the germ-layer theory to prove the essential unity of type of all the Triploblastica.
The Gastræa theory gave point and substance to the biogenetic law, and enabled Haeckel to state much more concretely the parallelism existing between ontogeny and phylogeny. He was able to assert that five primordial stages, each representing a primitive ancestral form, recurred with regularity in the very earliest development of all Metazoa.[437] These were the monerula, cytula, morula, blastula, and gastrula (see [Fig. 15]). The monerula was the fertilised ovum after the disappearance of the germinal vesicle;[438] it was the equivalent of the primordial anucleate Monera which are the ancestors of all animals.
Fig. 15.—The Five Primary Stages of Ontogeny. (After Haeckel.)
| 1. Monerula. | 2. Cytula. | 3. Morula. | 4. Blastula. | 5. Gastrula. |
The ovum after the nucleus had been re-formed became the cytula, which was the ontogenetic counterpart of the amœba. The morula, a compact mulberry-like congeries of segmentation-cells, corresponded to the synamœba, or earliest association of undifferentiated amœboid cells to form the first multicellular organism. The blastula, or hollow sphere of segmentation cells, usually ciliated, was reminiscent of the planæa, an ancestral free-swimming form whose nearest living relation is the spherical Magosphæra. The gastrula, finally, is the two-layered sac formed from the blastula, typically by invagination of its wall. It repeats the organisation of the gastræa, which is the common ancestor of all Metazoa, and finds its nearest living counterpart in the simple "sponges" Haliphysema and Gastrophysema.[439] The ancestral line of all the higher animals begins with the five hypothetical forms of the moneron, amœba, synamœba, planæa, and gastræa.
We may take the following account[440] of the phylogeny of the human species, from the gastræa stage onwards, as typical of Haeckel's speculations on the evolution of the higher forms. The progenitors of man are, after the Gastræada:—
1. Turbellaria.
*2. Scolecida. (Worms with a cœlom, probably represented at the present day by Balanoglossus.)
*3. Himatega. (Evolved from Scolecida by formation of dorsal nerve-tube and chorda, and resembling tailed larvæ of Ascidians.)
4. Acrania. (With metameric segmentation. Including Amphioxus.)
5. Monorrhina. (Cyclostomes.)
6. Selachia.
7. Dipneusta.
8. Sozobranchia. (Amphibia with permanent gills.)
*10. Protamnia.
*11. Promammalia.
12. Marsupialia.
13. Prosimiæ.
14. Menocerca. (Tailed apes.)
15. Anthropoides.
16. Pithecanthropi.
17. Homines.
It will be noticed that except for the hypothetical forms (marked with an asterisk), which are themselves generalised classificatory groups, the ancestral forms belong to long-recognised classes. The whole course of the evolution follows well-worn systematic lines. This is typical of Haeckel's phylogenetic speculations.
A more abstractly morphological scheme of the evolution of Vertebrates is given in the Systematic Phylogeny of 1895.[441] The ontogenetic and ancestral stages are arranged in parallel columns thus:—
| Cytula. | Cytæa (Protozoa). | ||
| Morula. | Moræa (Cœnobium ofProtozoa). | ||
| Blastula. | Blastæa (Volvocina,etc.). | ||
| Depula (invaginated blastula). | Depæa. | ||
| Gastrula. | Gastræa (cf. Olynthus,Hydra, and primitive Coelentera). | ||
| Cœlomula (with one pair ofcœlom-pockets). | Cœlomæa (cf. Sagitta,Ascidia, and primitive Helminthes). | ||
| Chordula (with medullary tube andchorda). | Chordæa (cf. Ascidian larvaand larva of Amphioxus). | ||
| Spondula (with segmented mesoderm). | Prospondylus (Primitive Vertebrate). | ||
This scheme differs from the earlier one chiefly in taking into account certain advances, notably as regards the cytology of the fertilised ovum and the true nature of the cœlom, which had been made in the interval of some twenty years.
Haeckel's Gastræa theory, though it exercised a great influence upon the subsequent trend of phylogenetic speculation, was by no means universally accepted telle quelle. Opinions differed considerably as to the primitive mode of origin of the two-layered sac which was very generally admitted to be of constant occurrence in early embryogeny. Ray Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a "Planula" theory, according to which the ancestral form of the Metazoa was a two-layered closed sac formed typically by delamination, less often by invagination. He denied that the invagination opening (which he named the blastopore) represented the primitive mouth,[443] holding that this was typically formed by an "inruptive" process at the anterior end of the planula, which led to the formation of a "stomodæum." A similar process at the posterior end gave rise to the anus and the "proctodæum."
The question as to whether delamination or invagination was to be considered the more primitive process was discussed in detail by Balfour,[444] without, however, any very definite conclusion being reached. He held that both processes could be proved in certain cases to be purely secondary or adaptive, and that accordingly there was nothing to show that either of them reproduced the original mode of transition from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by no means rejected the theory that the Gastræa, "however evolved, was a primitive form of the Metazoa," but, having regard to the great variations shown in the relation of the blastopore to mouth and anus (pp. 340-1), he was inclined to think that if the gastrula had any ancestral characters at all, these could only be of the most general kind. Balfour's attitude perhaps best represents the general consensus of opinion with regard to the Gastræa theory.
From the same origins as the Gastræa theory arose the theory of the cœlom. The term dates back to Haeckel in 1872, and the observations which first led up to the theory were made by the men who supplied the foundations of the Gastræa theory—A. Agassiz, Metschnikoff and Kowalevsky. But it was not Haeckel himself who enunciated the cœlom theory.
It will be remembered that Remak introduced in 1855 the conception of the mesoderm as an independent layer derived from the endoderm. The pleuro-peritoneal or body-cavity was formed as a split in the "ventral plates" of the mesoderm. Haeckel's "cœlom" corresponded to the "pleuro-peritoneal cavity" of Remak, but his view of the origin of the mesoderm brought him much closer to von Baer's conception of the origin of two secondary layers from ectoderm and endoderm respectively than to Remak's conception of the mesoderm as a single independent layer.
Much uncertainty reigned at the time as to the exact manner of origin of the mesoderm;[445] some held that it developed from the ectoderm, others that it originated in the endoderm, while still others, and among them Haeckel, considered that part of it came from the ectoderm and part from the endoderm (pp. 23-4, 1874).
The solution of the problem came from those observations on the development of the lower forms to which we have just alluded.
The early history of these discoveries and of the theory which grew out of them has been well summarised by Lankester,[446] and may conveniently be given in his own words:—
"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish," in Contributions to the Natural History of the United States, vol. v., 1864) showed in his account of the development of Echinoderma that the great body-cavity of those animals developed as a pouch-like outgrowth of the archenteron of the embryo, whilst a second outgrowth gave rise to their ambulacral system; and in 1869 Metschnikoff (Mém. de l'Acad. impériale des Sciences de St Pétersbourg, series vii., vol. xiv., 1869), confirmed the observations of Agassiz, and showed that in Tornaria (the larva of Balanoglossus) a similar formation of body-cavities by pouch-like outgrowths of the archenteron took place. Metschnikoff has further the credit of having, in 1874 (Zeitsch. wiss. Zoologie, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the relationship of the cœlenteric apparatus of the Enterocœla to the digestive canal and body-cavities of the higher animals. Leuckart had in 1848 maintained that the alimentary canal and the body-cavity of higher animals were united in one system of cavities in the Enterocœla (Verwandschaftsverhältnisse der wirbellosen Thiere, Brunswick, 1848). Metschnikoff insisted upon such a correspondence when comparing the Echinoderm larva, with its still continuous enteron and cœlom, to a Ctenophor, with its permanently continuous system of cavities and canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was formed by a division of the archenteron into three parallel cavities, and in 1874 demonstrated the same fact for the Brachiopoda. In 1875 (Quart. Journ. Micr. Sci., vol. xv., p. 52) Huxley proposed to distinguish three kinds of body-cavity: the schizocœl, formed by the splitting of the mesoblast, as in the chick's blastoderm; the enterocœl, formed by pouching of the archenteron, as in Echinoderms, Sagitta and Brachiopoda; and the epicœl.... Immediately after this I put forward the theory of the uniformity of origin of the cœlom as an enterocœl (Quart. Journ. Micr. Sci., April, 1875).... My theory of the cœlom as an enterocœl was accepted by Balfour and was greatly strengthened by his observations on the derivation of both notochord and mesoblastic somites from archenteron in the Elasmobranchs, and by the publication in 1877 by Kowalevsky of his second paper on the development of Amphioxus—in which the actual condition which I had supposed to exist in the Vertebrata was shown to occur, namely, the formation of the mesoblast as paired pouches in which a narrow lumen exists, but is practically obliterated on the nipping-off of the pouch from the archenteron, after which process it opens out again as cœlom" (pp. 16-18).
The enterocœlic theory was taken up by O. and R. Hertwig as an essential part of their Cœlomtheorie.[447] In a lengthy series of monographs these workers made a comparative study of the mode of formation of the middle layer, and arrived at a coherent theory of its origin. They distinguished in the middle layer two quite distinct elements, the mesoblast proper, formed by the evagination of the walls of the archenteron, and the mesenchyme, formed by free cells budded off from the germ-layers. The following passage gives a good idea of their views and of the phylogenetic implications involved:—"Ectoblast and entoblast are the two primary germ-layers which arise from the invagination of the blastula; they are always the first to be laid down, and they can be directly referred back to a simple ancestral form, the Gastræa; they form the limits of the organism towards the exterior and towards the archenteron. The parietal and visceral mesoblast, or the two middle layers, are always of later origin, and arise through evagination or plaiting of the entoblast, the remainder of which can now be distinguished as secondary entoblast from the primary. They form the walls of a new cavity, the enterocœl, which is to be regarded as a nipped-off diverticulum of the archenteron. Just as the two-layered animals can be derived from the Gastræa, so can the four-layered animals be derived from a Cœlom form. Embryonic cells, which become singly detached from their epitheliar connections we consider to be something quite different from the germ-layers, and accordingly we call them by the special name of mesenchyme germs or primary cells of the mesenchyme. They may develop both in two-layered and in four-layered animals. Their function is to form between the epithelial limiting layers a secreted tissue (Secretgewebe) or connective tissue with scattered cells, which cells can undergo, like the epithelial elements, the most varied modifications.... This secreted tissue in its simple or in its differentiated state, with all its derivatives, we call the mesenchyme" (p. 122).
The important point for us is that, just as all Metazoa were considered by Haeckel to be descended from the Gastræa, so all Cœlomati were held by the Hertwigs to be derived from an original cœlomate Urform. In both cases an embryological archetype becomes a hypothetical ancestral form.
The Cœlom theory was considerably modified, extended and developed by later workers, particularly as regards the relations to the cœlom of the genital organs and ducts and the nephridia, but no special methodological interest attaches to these further developments.[448] We shall here focus attention upon one interesting line of speculation followed out in this country particularly by Sedgwick—the theory of the Actinozoan ancestry of segmented animals. Its relation to the Cœlom theory lies in the fact that Sedgwick regarded the segmentation of the body as moulded upon the segmentation of the mesoblast, which in its turn, as Kowalevsky and Hatschek had shown, was a consequence of its mode of origin as a series of pouches of the archenteron. In other respects Sedgwick's speculations link on more closely to the Gastræa theory, for one of his main contentions is that the blastopore or Urmund is homologous throughout at least the three metameric phyla. In following up Balfour's observations on the development of Peripatus,[449] Sedgwick was struck with the close resemblance existing between the elongated slit-like blastopore of this form (giving rise to both mouth and anus), with its border of nervous tissue, and the slit-like mouth of the Actinozoan (functioning both as mouth and anus), round which, as the Hertwigs had shown, there lies a special concentration of nerve cells and nerve fibres. He found another point of resemblance in the gastric pouches of the Actinozoa, which he homologised directly with the enterocœlic pouches of the Cœlomati. He was led to enunciate the following theses:—[450] (1) that the mouth and anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived from the elongated mouth of an ancestor resembling the Actinozoa; (2) that somites are derived from a series of archenteric pouches, like those of Actinozoa and Medusæ; (3) that excretory organs (nephridia, segmental organs) are derived from parts of these pouches which in the ancestral form, as in many polyps, were connected by a circular or longitudinal canal, and opened to the exterior by pores. This longitudinal canal was lost in Invertebrates, but persisted in Vertebrates as the pronephric duct, while the pores remained in Invertebrates and disappeared in Vertebrates; (4) that the tracheæ of Arthropods, as well as the canal of the central nervous system in Vertebrates, are to be traced back to certain ectodermal pits in the diploblastic ancestor comparable to the sub-genital pits of the Scyphomedusæ. These ectodermal pits were all originally respiratory organs. "The essence of all these propositions," he writes, "lies in the fact that the segmented animals are traced back not to a triploblastic unsegmented ancestor, but to a two-layered Cœlenterate-like animal with a pouched gut, the pouching having arisen as a result of the necessity for an increase in the extent of the vegetative surfaces in a rapidly enlarging animal (for circulation and respiration)" (p. 47). "I have attempted to show," he writes further on, "that the majority of the Triploblastica ... are built upon a common plan, and that that plan is revealed by a careful examination of the anatomy of Cœlenterata; that all the most important organ-systems of these Triploblastica are found in a rudimentary condition in the Cœlenterata; and that all the Triploblastica referred to must be traced back to a diploblastic ancestor common to them and the Cœlenterata" (p. 68). The main assumption was that the neural or blastoporal surface must be homologous throughout the Metazoa, though it was dorsal in the Chordata, ventral in the Annelida and Arthropoda. He derived the central nervous system of the Chordata from the circumoral ring of the common ancestor by means of the hypothesis that both the pre-blastoporal and the post-blastoporal parts of it disappeared.[451]
The characteristic relation of the central nervous system to the blastopore in Annelida and Vertebrates had already been pointed out by Kowalevsky,[452] who had also sketched a theory of the common descent of these two phyla from an ancestral form in which the nervous system encircled the blastopore.
In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put forward the somewhat similar view that the stomach-diverticula of the Turbellaria, which he had found to be segmentally arranged in certain Triclads, were the morphological equivalents of the enterocœlic pouches of higher animals. This view, however, he soon gave up.[454] Sedgwick's views found a supporter in A. A. W. Hubrecht,[455] who utilised them in connection both with his speculations on the relation of Nemertines to Vertebrates, and with his exhaustive work on the early development of the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an actinia-like, vermiform being, elongated in the direction of the mouth-slit" (p. 410, 1906), and derived the central nervous system from the circum-oral ring of this primitive form, the notochord from its stomodæum, and the cœlom from the peripheral parts of the gastric cavity (p. 169, 1909).
[424] Gegenbaur, Zeits. f. wiss. Zool., v., 1853.
[425] Remak, loc. cit., p. 183, pl. xii.
[426] Lereboullet, Ann. Sci. nat. (4) xviii., pp. 118-9, 1862.
[427] Lereboullet, in Remak, p. 183 f.n.
[428] Kowalevsky, Mém. Acad. Sci. St Pétersbourg (Petrograd), (7), x. and xi., 1866 and 1867.
[429] A. Agassiz, Contrib. Nat. Hist. United States, v., 1864.
[430] Mém. Acad. Sci. St Pétersbourg (Petrograd), (7), xiv., 1869.
[431] "Embryolog. Studien an Würmern u. Arthropoden," Mém. Acad. Sci. St Pétersbourg (Petrograd), (7), xvi., 1870.
[432] Die Kalkschwämme, 3 vols., Berlin, 1872. General chapters translated in Ann. Mag. Nat. Hist. (4), xi., pp. 241-62, 421-30, 1873.
[433] "Die Gastræa-Theorie, die phylogenetische Classification des Thierreichs und die Homologie der Keimblätter." Jenaische Zeitschrift, viii., pp. 1-55, 1874. "Die Gastrula und die Eifurchung der Thiere," ibid., ix., pp. 402-508, 1875. "Die Physemarien, Gastræaden der Gegenwart," and "Nachträge zur Gastræa-Theorie," ibid., x., pp. 55-98, 1876. Republished in Biologische Studien, 2nd part, Studien zur Gastræa-Theorie, 270 pp., 14 pls., Jena, 1877.
[434] See Ann. Mag. Nat. Hist. (4), xi., p. 253.
[435] Term first introduced in Die Kalkschwämme, p. 468, 1872.
[436]"On the Primitive Cell-layers of the Embryo as the Basis of Genealogical Classification of Animals, and on the Origin of Vascular and Lymph Systems," Ann. Mag. Nat. Hist. (4), xi., pp. 321-38, 1873.
[437] First distinguished in Die Kalkschwämme, i., p. 465.
[438] Even in the 'seventies it was still believed by many that the egg-nucleus disappeared on fertilisation. The true nature of the process was not fully made out till 1875, when O. Hertwig observed the fusion of egg- and sperm-nuclei in Toxopneustes (Morph. Jahrb., i., 1876).
[439] Studien z. Gastræa-Theorie, p. 214, 1877. These forms were known even in 1870 (Carter, Ann. Mag. Nat. Hist. (4), vi., pp. 346-7), to be Foraminifera. The figures of supposed collar-cells, etc., do credit to Haeckel's imagination.
[440] History of Creation, Eng. Trans., ii., pp. 278 ff.
Systematische Phylogenie, iii., p. 41, Berlin, 1895.
[442] "Notes on the Embryology and Classification of the Animal Kingdom," Q.J.M.S. (n.s.), xvii., pp. 399-454, 1877.
[443] It was "part of the non-historic mechanism of growth" (loc. cit., p. 418).
[444] Treatise on Comparative Embryology, ii., chap. xiii., 1881. For a modern discussion of this problem, see Hubrecht, Q.J.M.S., xlix., 1906.
[445] See Balfour, loc. cit., Chapter xiii.
[446] A Treatise on Zoology, Pt. ii., 1900. Introduction by Sir E. Ray Lankester.
[447] Studien zur Blättertheorie, Jena, 1879-80. "Die Cœlomtheorie, Versuch einer Erklärung des mittleren Keimblattes," Jenaische Zeitschrift, xv., pp. 1-150, 1882.
[448] For an historical account of this work, see Lankester, loc. cit., pp. 21-37.
[449] Proc. Roy. Soc., 1883, and Q.J.M.S., xxiii., 1883.
[450] "Origin of Metameric Segmentation," Q.J.M.S., xxiv., pp. 43-82 1884.
[451] See further the same author's article "Embryology" in the Ency. Brit., vol. xi., 11th ed., Cambridge, 1910.
[452] Arch. f. mikr. Anat., xiii., pp. 181-204, 1877.
[453] "Der Bau von Gunda segmentata," Mitth. Zool. Stat. Neap., iii., pp. 187-250, 1882.
[454] "Die Polycladen," Fauna u. Flora des Golfes von Neapel, Monog. v., Leipzig, 1884, and "Beiträge zu einer Trophocœltheorie," Jen. Zeits., xxxviii., pp. 1-373, 1904 (which see for a modern account of theories of metamerism).
[455] "Die Abstammung der Anneliden u. Chordaten," Jen. Zeits., xxxix., pp. 151-76, 1905. "The Gastrulation of the Vertebrates," Q.J.M.S., xlix., pp. 403-19, 1906. "Early Ontogenetic Phenomena in Mammals," Q.J.M.S., liii., pp. 1-181, 1909.