THE EPIGENETICS OF THE EMBRYONIC AXIS 



191 



it is as though the mesoderm of the amphibian gastrula has the same kind 

 of epigenetic organisation as the whole newly fertilised egg of the sea- 

 urchin, the chorda corresponding to the high point of the vegetative 

 gradient and the most ventral mesoderm to the animal pole. Dalcq and 

 Pasteels (1937, 1938, Dalcq 1941) have particularly emphasised this 

 gradient system within the mesoderm, and suggest that it is derived from 

 the two components which were active at the time of formation of the 

 grey crescent, namely, a general gradient in yolk content extending 





^j^/ 



Figure 10. 10 



Differentiation of various regions of the flank mesoderm of the neurula. 

 Column I illustrates their presumptive fate; column 2 what they produce 

 when isolated; column 3 what they develop into when combined with a 

 fragment of presumptive notochord. The tissues represented are blood 

 cells, nephric tubules, muscle and notochord. (From Yamada 1940). 



through the whole egg from the animal pole to the point of highest con- 

 centration at the vegetative pole, and a cortical gradient of unknown 

 nature, located in the outermost layer of cytoplasm and with its highest 

 point at the position where the dorsal lip first appears. The Belgian authors 

 show that one can derive a formal explanation of many phenomena of 

 early development from the interaction of these two postulated gradients; 

 but the concept rather lacks precision when applied to these compara- 

 tively late stages of development (see the criticism of Rotmann 1943). 



The individuation of an organ or of a whole embryonic axis must be 

 highly complex process in which the various parts of the mass of tissue 

 interact with one another in several different types of induction process. 

 Each individual biochemical interaction can, perhaps, be regarded as an 



