234 



Embryogenesis: Progressive Differentiation 



INTERPRETATION OF DORSALIZATION 

 IN TERMS OF GRADIENTS 



The occurrence of blastopores outside of 

 the gray crescent area has led some authors 

 to propose a more dynamic interpretation of 

 the origin of dorsalization. Penners and 

 Schleip ('28a,b) have contended that the 

 essential condition for blastopore formation 

 is a gradient between dark cortical material 

 and white yolk, and that invagination would 

 occur at that point of the egg surface at which 

 this gradient is steepest. Dalcq and Pasteels 

 ('37, '38; Dalcq, '41a,b) have formulated an 

 elaborate double-gradient theory which en- 

 compasses not only the patterning of morpho- 

 genetic movements and tissue segregation, 

 but also the subsequent differentiations and 

 inductions. Only a few points of this theory 

 are relevant to the present discussion. 



The authors postulate (1) a "yolk-gradient 

 field" with its center at the vegetal pole; it 

 is not necessarily represented by yolk, but 

 more likely by the cytoplasm combined with 

 the yolk; (2) a "cortical gradient field" which 

 resides in the cortex; it has its center in the 

 center of the gray crescent and extends 

 around the entire surface of the egg. Both 

 fields show continuous decrements and both 

 elaborate specific products, V (vitelline) and 

 C (cortical), respectively, which interact 

 with each other to produce a new compound, 

 "organisine." Every region on the egg surface 

 is defined by two variables: the product C 

 X V, and the ratio C/V. The basic point of 

 the theory is the following: The initial egg 

 organization is represented by merely quanti- 

 tative regional differences between the two 

 variables, and these differences can be 

 modified experimentally. The region of the 

 peak vahie of C X F becomes the site of 

 blastopore formation and mesodermization. 

 In the normal egg, this peak would be at- 

 tained in the area halfway between the 

 centers of the two gradient fields; but the 

 experimental dislocation of the more mobile 

 yolk-gradient field by centrif ligation or in- 

 version may raise the C X V value of any 

 other point on the egg surface to a sufficientlv 

 high level to endow it with the capacities of 

 invagination and mesodermal differentiation. 

 The experimental support of this theorv can- 

 not be presented here (see reviews in Dalcq, 

 '41a; Pasteels, '51). It is largely based on 

 the inversion experiments of Pasteels ('38, 

 '40) in which the positions of the two fields 

 with respect to each other and their inter- 

 actions could be controlled experimentally. 



A number of criticisms have been levelled 



against this hypothesis (see Rotmann, '43; 

 Lehmann, '45). The question of whether the 

 yolk-cytoplasm system exerts its influence by 

 producing a chemical substance or merely 

 by providing a favorable mechanical matrix 

 for mesodermal invagination is still open. 

 Furthermore, the significance of the sub- 

 cortical "marginal plasma" for the processes 

 under discussion remains to be settled. 

 Lehmann ('42a, '45) ascribes to it an im- 

 portant role, whereas Pasteels ('46) denies its 

 existence. According to Pasteels ('51), the 

 subcortical marginal zone is characterized 

 by a special ratio of yolk platelets and 

 basophilic granules. None of the cytological 

 pictures presented so far permit definite con- 

 clusions, and detailed studies of serial sec- 

 tions of eggs immediately after rotation and 

 centrifugation are altogether missing. There- 

 fore, mvich of the discussion is of a highly 

 speculative nature. A more detailed study of 

 the egg structure with modern tools will be 

 necessary before a decision between the two 

 hypotheses becomes possible. 



GASTRULATION, FATE MAPS 



Gastrulation is a very significant event 

 in vertebrate development. In this phase, 

 the body plan is created and the main organ 

 primordia are blocked out and brought into 

 their definite spatial relations to each other. 



The classic vital staining experiments of 

 Vogt ('29) have shown that gastrulation in 

 amphibians is accomplished by complex 

 morphogenetic movements which are inte- 

 grated in space and time. His investigations 

 on urodeles and anurans have clarified 

 nvimerous controversial issues and have laid 

 the foundation for an understanding of gas- 

 trulation in meroblastic vertebrates (see 

 Pasteels, '37b). Space does not permit us to 

 give an adequate presentation of these phe- 

 nomena; we refer to textbooks on comparative 

 and experimental embryology. A detailed ac- 

 count of Vogt's work may be found in Ham- 

 burger ('42). 



The transformation of the blastula into the 

 thi-ee-layered gastrula is accomplished by 

 an invagination of the lower hemisphere 

 which includes the marginal zone (prospec- 

 tive mesoderm) and the vegetal material 

 (prospective entoderm), and an expansion 

 (epiboly) of the upper hemisphere (prospec- 

 tive ectoderm). Invagination begins at the 

 dorsal lip of the blastopore, where at first 

 pharyngeal entoderm, then prechordal meso- 

 derm and chorda material move inward; 

 somewhat later, invagination of lateral and 



