If the single phases of differentiation are to be regarded as effects, we must ask for the causes, or stimuli, of these effects. For a full account of the subject we refer to Herbst, by whom also the whole botanical literature, much more Formative stimuli. important than the zoological, is critically reviewed. We have already seen that when the blastula represents an aequipotential system, there must be some sort of primary organization of the egg, recoverable after disturbances, that directs and localizes the formation of the first embryonic organs; we do not know much about this organization. Directive stimuli (Richtungsreize) play a great role in ontogeny; Herbst has analysed many cases where their existence is probable. They have been experimentally proved in two cases. The chromatic cells of the yolk sac of Fundulus are attracted by the oxygen of the arteriae (Loeb); the mesenchyme cells of Echinus are attracted by some specific parts of the ectoderm, for they move towards them also when removed from their original positions to any point of the blastocoel by shaking (Driesch). Many directive stimuli might be discovered by a careful study of grafting experiments, such as have been made by Born, Joest, Harrison and others, but at present these experiments have not been carried out far enough to get exact results.
Formative stimuli in a narrower meaning of the word, i.e. stimuli affecting the origin of embryonic organs, have long been known in botany; in zoology we know (especially from Loeb) a good deal about the influence of light, gravitation, contact, &c., on the formation of organs in hydroids, but these forms are very plant-like in many respects; as to free-living animals, Herbst proved that the formation of the arms of the pluteus larva depends on the existence of the calcareous tetrahedra, and made in other cases (lens of vertebrate eye, nerves and muscles, &c.) the existence of formative stimuli very probable. Many of the facts generally known as functional adaptation (functionelle Anpassung—Roux) in botany and zoology may also belong to this category, i.e. be the effects of some external stimulus, but they are far from having been analysed in a satisfactory manner. That the structure of parts of the vertebrate skeleton is always in relation to their function, even under abnormal conditions, is well known; what is the real “cause” of differentiation in this case is difficult to say.
It is obvious that we cannot answer the question why the different ontogenetic effects are just what they are. Developmental physiology takes the specific nature of form for granted, and it may be left for a really rational theory Specific characters. of the evolution of species in the future to answer the problem of species, as far as it is answerable at all. What we intend to do here is only to say in a few words wherein consists the specific character of embryonic organs. That embryonic parts are specific or typical in regard to their protoplasm is obvious, and is well proved by the fact that the different parts of the embryo react differently to the same chemical or other reagents (Herbst, Loeb). That they may be typical also in regard to their nuclei was shown by Boveri for the generative cells of Ascaris; we are not able at present to say anything definite about the importance of this fact. The specific nature of an embryonic organ consists to a high degree in the number of cells composing it; it was shown for many cases that this number, and also the size of cells, is constant under constant conditions, and that under inconstant conditions the number is variable, the size constant; for instance, embryos which have developed from one of the two first blastomeres show only half the normal number of cells in their organs (Morgan, Driesch).
We have learnt that the successive steps of embryonic development are to be regarded as effects, caused by stimuli, which partly exist in the embryo itself. But it must be noted that not every part of the embryo is dependent on every Self-differentiation. other one, but that there exists a great independence of the parts, to a varying degree in every case. This partial independence has been called self-differentiation (Selbstdifferenzierung) by Roux, and is certainly a characteristic feature of ontogeny. At the same time it must not be forgotten that the word is only relative, and that it only expresses our recognition of a negation.
For instance, we know that the ectoderm of Echinus may develop further if the endoderm is taken away; in other words, that it develops by self-differentiation in regard to the endoderm, that its differentiation is not dependent on the endoderm; but it would be obviously more important to know the factors on which this differentiation is actually dependent than to know one factor on which it is not. The same is true for all other experiments on “self-differentiation,” whether analytical (Loeb, Schaper, Driesch) or not (grafting experiments, Born, Joest, &c.).
Can we understand differentiation by means of the laws of natural phenomena offered to us by physics and chemistry? Most people would say yes, though not yet. Driesch has tried to show that we are absolutely not able to Vitalism. understand development, at any rate one part of it, i.e. the localization of the various successive steps of differentiation. But it is impossible to give any idea of this argument in a few words, and we can only say here that it is based on the experiments upon isolated blastomeres, &c., and on an analysis of the character of aequipotential systems. In this way physiology of development would lead us straight on into vitalism.
References.—An account of the subject, with full literature, is given by H. Driesch, Resultate und Probleme der Entwicklungsphysiologie der Tiere in Ergebnissen der Anat. u. Entw.-Gesch. (1899). Other works are: C.H. Davenport, Experimental Morphology (New York, 1897-1899); Y. Delage, La Structure du protoplasma, &c. (1895); Driesch, Mathem. mech. Betrachtung morpholog. Probleme (Jena, 1891); Entwicklungsmechan. Studien (1891-1893); Analytische Theorie d. organ. Entw. (Leipzig, 1894); Studien über d. Regulationsvermögen (1897-1900), &c.; C. Herbst, “Über die Bedeutung d. Reizphysiologie für die kausale Auffassung von Vorgängen i. d. tier. Ontogenese,” Biolog. Centralblatt, vols. xiv. u. xv. (Leipzig, 1894). Many papers on influence of salts on development in Arch. f. Entw.-Mech.; O. Hertwig, Papers in Arch. f. mikr. Anat., “Die Zelle und die Gewebe,” ii. (Jena, 1897); W. His, Unsere Körperform (Leipzig, 1875); J. Loeb, Untersuch. z. physiol. Morph. (Würzburg, 1891-1892). Papers in Arch. f. Entw.-Mech. and Pflüger’s Archiv; T.H. Morgan, The Development of the Frog’s Egg (New York, 1897); Papers in Arch. f. Entw.-Mech.; Roux, Gesammelte Abhandlungen (Leipzig, 1895); Papers in Arch. f. Entw.-Mech.; A. Weismann, Das Keimplasma (Jena, 1892); E.B. Wilson, papers in Journ. Morph., “The Cell in Development and Inheritance” (New York, 1896).
(H. A. E. D.)
[1] In the mammalia the word foetus is often employed in the same signification as embryo; it is especially applied to the embryo in the later stages of uterine development.