II KINETIC AND CAUSATIVE ASPECTS 347 



Nevertheless, the hps obtained by ligature may be quite efficient and typical morpho- 

 choresis can develop there, with the production of notochord, somites, etc. This could 

 suggest that the kinematic processes, once initiated in a given center, are by themselves 

 sufficient to initiate the ultimate production of the axial organs without any former 

 prolonged elaboration. This idea, however, does not hold in all cases. Other experiments 

 performed on Discoglossus eggs, at the same stage, have shown that the presumptive areas 

 are not only a topographic expression of the results obtained by vital staining, but that 

 the main territories are endowed with intrinsic, although labile, properties, analogous to 

 Vogt's 5fl/z«?/n^ or "predispositions" (Dalcq, 1940, 1941a). Thus, in the case of the ligatures of 

 Huang, the operation seems to have produced a local increase of morphogenetic potential 

 in the presumptive marginal material which is involved. 



Results obtained in teleost eggs would be too complicated to describe in detail. Let us 

 recall here the elementary, but important fact, that the embryogenic area is at first a 

 limited cap of pure cytoplasm, and that it rapidly increases by a flow of material squeezed 

 from the yolk. In certain eggs, such as the zebra fish (Lewis and Roosen-Runge, 1943), 

 this flow is repeated alternately with mitosis. Latitudinal sections, which isolate the 

 blastodisc at various stages, give results which diflfer according to the species, to the importance 

 of the material already incorporated in the blastodisc, and to the nature of the culture 

 medium used (Devillers, 1949; Trinkhaus, 1953; Tung, 1955). 



There are hints that active materials, originating from the yolk, become located in a 

 definite part of the periblast, and cause this side to become dorsal. Such interpretation 

 has been proposed by Tung et al. (1945) for the gold-fish egg, and tested on the trout by 

 Devillers (1952). This rather complex experiment consisted of exchanging dorsal and 

 ventral half of blastodiscs; the results were in general agreement with Tung's hypothesis. 

 Thus, these data are favorable to the general idea of an effective interaction between the 

 cortex and the yolk material. 



[d) Interpretation of kine?natics in amniots 



How far this concept can be applied to reptiles or birds has not been ascertained ; it 

 can only be said that the similarity, existing in later processes, suggests that a similar 

 mechanism may be operating at the youngest stages. In placentar mammals, the 

 situation is apparently different. No real yolk is available, and therefore no definite 

 interaction between a cortex and an endoplasm can be hypothetized. The first steps 

 of cleavage, already mentioned, have secondarily acquired a special modification, 

 the meaning of which, as an adaptation to placental viviparism, has been examined 

 elsewhere (Dalcq, 1954b). Why an endophyllic layer is produced by the enveloping 

 layer (at least in rodents) remains enigmatic. We can only remark that this pre- 

 gastrular kinematics is accompanied by an appearance in the ectophyllic material 

 of a considerable amount of RNA and alkaline phosphatase, both located in similar 

 granules (Fig. 17, p. 326 and Fig. 37, p. 364). During this period, the germ is 

 already in full growth, and the assimilated metabolites are largely used to build 

 up the 2-layer stage, especially the ectophyll. It remains difficult to guess the cause 

 of the appearance of the primitive streak, and why it should be located along a 

 given radius. An inducing action of the endophyll can be suggested, but the idea 

 remains conjectural. 



In spite of their incompleteness, the information available on the appearance, 

 causation, and realization of morphochoresis justifies the general idea that the 

 field-gradient system of the fertilized egg gradually transforms and matures 

 during cleavage. In this way, in a vertebrate egg, a region of special importance 

 gradually becomes the so-called marginal zone, which includes mainly the 

 material destined to become notochord^ mesoblast and part of the endoblast. 



Literature p. 483 



