342 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



moment, expresses in this way its individuality. Of course, there is no sharp 

 distinction, at the beginning, between the types of deformation exhibited by 

 different groups. However, as mapping by vital staining shows, the final and 

 total result is that discontinuities appear, because — at the limit of two territories — 

 cells on one side go one way, and those on the other side, the other way. The 

 individuality of the primordial organs is soon strengthened by the acquisition 

 of new surface properties, which cause either adhesiveness or non-adhesiveness of 

 cells. Since, in the main trend of kinetics, we have to do with a conflict of divergent 

 forces, it is no wonder that the presumptive territories each have a definite shape^, 

 limited by straight or curved lines (Vogt, 1929; Pasteels, 1936, 1937, 1940; 

 Nakamura, 1938; Spratt, 1942; Malan, 1953). It is no wonder either, that the 

 movements accomplished by the various primordia not only transpose the cell- 

 groups as such, but also deform and remould them in different directions. The 

 real picture depends on the intrinsic properties and general configuration of the 

 germinal material at the onset, and on the more or less condensed character of the 

 cell streams. Patient and exact work has now resulted in valuable descriptive 

 accounts for all groups; they may serve as a basis for comparisons which reveal 

 the uniform dominants of kinetics : epiboly, invagination or immigration, dorsal 

 convergence, and cephalo-caudal stretching. For our present purpose, this general 

 sketching of the processes seems sufficient. 



(b) Enumeration of the problems 



What we are particularly concerned with here is a possible explanation of the very 

 fact that kinematics begins, follows a definite course, achieves the separation of the 

 layers and of the primordial organs. Let us consider the blastula stage of a fish or an 

 amphibian. Why does a blastopore appear, underlined by a groove progressively 

 deepening into an archenteron? Why does the middle layer soon produce the chorda, 

 the somites, the nephrotomes and the lateral plate, all names which imply a typical 

 fate? If we consider the reptile egg, why does not gastrulation begin at the margin of 

 the blastoderm but in some inner, slightly eccentric region? If we examine a bird egg, 

 why does an endophyllic layer at first creep under the superficial layer, and then 

 the primitive streak does arise along a radius of the oblong blastoderm? If we follow a 

 mammalian blastocyst, of a rodent for convenience, why do the enveloping cells {cf. 

 Fig. 1 7 k, p. 326) first provide an endophyll layer by immigration, so that a two- 

 layer stage is attained, which will proceed in a manner similar to bird development? 



We are far from having formed definite answers to these querries. For the last 

 three, so far no experiment has been attempted and we have no better resource 

 than comparative embryology, which is necessarily conjectural. However, it 

 seems plausible that if the case of some Anamniota could be solved, we would have 

 a clue to a more general interpretation. 



^ It has been claimed that in eggs of Xenopus the typical pattern of dorso-marginal areas 

 does not exist, an exception capable of invalidating the above considerations. However, 

 the contention of Nieuwkoop and Florschiitz (1950) is based only on the study of sections. 

 Sirlin (1956) supported the conclusions of these authors on the basis of labelled grafts, 

 but this argument is also questionable. Damas (1957) has applied vital staining to the same 

 eggs and maintains that a superficial chordo-mesoblastic area exists, as in all other species. 



