Fig. 382—Transverse section of the cardiac region of the same chick-embryo (behind the preceding). In the cervical cavity (hh) the heart (h) is still connected by a mesocard (hg) with the gut-fibre layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary auditory vesicle in the horny plate, hp first rise of the amniotic fold. (From Remak.)
There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice. In its earliest form the heart is double, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we took as our starting-point, is only formed at a later stage, when the two lateral tubes move backwards, touch each other, and at last combine in the middle line. In man, as in the rabbit, the two embryonic hearts are still far apart at the stage when there are already eight primitive segments (Fig. 134 h). So also the two cœlom-pouches of the head in which they lie are still separated by a broad space. It is not until the permanent body of the embryo develops and detaches from the embryonic vesicle that the separate lateral structures join together, and finally combine in the middle line. As the median partition between the right and left cardiocœl disappears, the two cervical cavities freely communicate (Fig. 381), and form, on the ventral side of the amniote head, a horseshoe-shaped arch, the points of which advance backwards into the pleuro-ducts or pleural cavities, and from there into the two peritoneal sacs of the trunk. But even after the conjunction of the cervical cavities (Fig. 381) the two cardiac tubes remain separate at first; and even after they have united a delicate partition in the middle of the simple endothelial tube (Figs. 379 s, 382 h) indicates the original separation. This cenogenetic “primary cardiac septum” presently disappears, and has no relation to the subsequent permanent partition between the halves of the heart, which, as a heritage from the reptiles, has a great palingenetic importance.
Thorough opponents of the biogenetic law have laid great stress on these and similar cenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law. As in every other instance, careful, discriminating, comparative-morphological examination converts these supposed disproofs of evolution into strong arguments in its favour. In his excellent work, On the structure of the Heart in the Amphibia (1886), Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk.
The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny. But as one needs a thorough knowledge of the intricate structure of the whole vascular system in man and the other Vertebrates in order to follow this with profit, we cannot go into it further here. Moreover, many important features in the ontogeny of the vascular system are still very obscure and controverted. The characters of the embryonic circulation of the Amniotes, which we have previously considered (Chapter XV), are late acquisitions and entirely cenogenetic. (Cf. pp. 170–171; Figs. 198–202.)
Chapter XXIX.
EVOLUTION OF THE SEXUAL ORGANS
If we measure the importance of the systems of organs in the animal frame according to the richness and variety of their phenomena and the physiological interest that this implies, we must regard as one of the principal and most interesting systems the one which we are now going to examine—the system of the reproductive organs. Just as nutrition is the first and most urgent condition for the self-maintenance of the individual organism, so reproduction alone secures the maintenance of the species—or, rather, the maintenance of the long series of generations which the totality of the organic stem represents in their genealogical connection. No individual organism has the prerogative of immortality. To each is allotted only a brief span of personal development, an evanescent moment in the million-year course of the history of life.
Hence, reproduction and the correlative phenomenon, heredity, have long been regarded, together with nutrition, as the most important and fundamental function of living things, and it has been attempted to distinguish them from “lifeless bodies” on this very score. As a matter of fact, this division is not so profound and thorough as it seems to be, and is generally supposed to be. If we examine carefully the nature of the reproductive process, we soon see that it can be reduced to a general property that is found in inorganic as well as organic bodies—growth. Reproduction is a nutrition and growth of the organism beyond the individual limit, which raises a part of it into the whole. This is most clearly seen when we study it in the simplest and lowest organisms, especially the Monera (Figs. 226–228) and the unicellular Amœbæ (Fig. 17). There the simple individual is a single plastid. As soon as it has reached a certain limit of size by continuous feeding and normal growth, it cannot pass it, but divides, by simple cleavage, into two equal halves. Each of these halves then continues its independent life, and grows on until it in turn reaches the limit of growth, and divides. In each of these acts of self-cleavage two new centres of attraction are formed for the particles of bodies, the foundations of the two new-formed individuals. There is no such thing as immortality even in these unicellulars. The individual as such is annihilated in the act of cleavage (cf. p. 48).
In many other Protozoa reproduction takes place not by cleavage, but by budding (gemmation). In this case the growth that determines reproduction is not total (as in segmentation), but partial. Hence in gemmation also we may oppose the local growth-product, that becomes a new individual in the bud, as a child-organism to the parent-organism from which it is formed. The latter is older and larger than the former. In cleavage the two products are equal in age and morphological value. Next to gemmation we have, as other forms of asexual reproduction, the forming of embryonic buds and the forming of embryonic cells. But the latter leads us at once to sexual generation, the distinctive feature of which is the separation of the sexes. I have dealt fully with these various types of reproduction in my History of Creation (chap. viii) and my Wonders of Life (chap. xi).
The earliest ancestors of man and the higher animals had no faculty of sexual reproduction, but multiplied solely by asexual means—cleavage, gemmation, or the formation of embryonic buds or cells, as many Protozoa still do. The differentiation of the sexes came at a later stage. We see this most plainly in the Protists, in which the union of two individuals precedes the continuous cleavage of the unicellular organism (transitory conjugation and permanent copulation of the Infusoria). We may say that in this case the growth (the condition of reproduction) is attained by the coalescence of two full-grown cells into a single, disproportionately large individual. At the same time, the mixture of the two plastids causes a rejuvenation of the plasm. At first the copulating cells are quite homogeneous; but natural selection soon brings about a certain contrast between them—larger female cells (macrospores) and smaller male cells (microspores). It must be a great advantage in the struggle for life for the new individual to have inherited different qualities from the two cellular parents. The further advance of this contrast between the generating cells led to sexual differentiation. One cell became the female ovum (macrogonidion), and the other the male sperm-cell (microgonidion).
The simplest forms of sexual reproduction among the living Metazoa are seen in the Gastræads p. 233, the lower sponges, the common fresh-water polyp (Hydra), and other Cœlenteria of the lowest rank. Prophysema (Fig. 234), Olynthus (Fig. 238), Hydra, etc., have very simple tubular bodies, the thin wall of which consists (as in the original gastrula) only of the two primary germinal layers. As soon as the body reaches sexual maturity, a number of the cells in its wall become female ova, and others male sperm-cells: the former become very large, as they accumulate a considerable quantity of yelk-granules in their protoplasm (Fig. 235 e); the latter are very small on account of their repeated cleavage, and change into mobile cone-shaped spermatozoa (Fig. 20). Both kinds of cells detach from their source of origin, the primary germinal layers, fall either into the surrounding water or into the cavity of the gut, and unite there by fusing together. This is the momentous process of fecundation, which we have examined in Chapter VII (cf. Figs. 23–29).