104 



THE TOPOGENESIS OF THE EMBRYO 



problem of the causation of the different topogenetic potencies 

 in different parts of the blastula. It has been known for a long 

 time that the gastrulation of amphibian eggs is often disturbed 

 if, soon after fertilisation, the eggs are fixed with the animal 

 pole downwards (Schultze, 1894), (of. p. 48). Schleip and 

 Penners (1925-28), and Motomura (1935) repeated this ex- 



Fig. 38. Diagram of the co-operation of the gradients in the 

 development of amphibians. The cortical factor C decreases from 

 dorsal side (left in the figure) to ventral side (on the right), accord- 

 ing to line a . The yolk factor V decreases from the vegetative 

 pole (below) to the animal pole (above), according to line Q- 

 The product (7 X V is maximal at F (centre of the dorsal marginal 

 zone), and decreases gradually from there. The lines a, b, c, d, and 

 e indicate planes in which the product has the same value. The lines 

 HI and KL are the boundaries of areas in which the ratio C/V has 

 the same value. After Dalcq and Pasteels. 



periment, and finally Pasteels gave a satisfactory analysis of 

 the phenomenon (1938-39). It appeared that in such eggs a 

 blastopore may be formed by invagination at any point where 

 yolk material is in direct contact with the egg surface, but 



