90 



CLEAVAGE AND DIFFERENTIATION 



this differentiation is of such a kind that a certain cytoplasmic 

 region is essential for development. There is no other qualitative 

 difference between the blastomeres of the 2- or 4-cell stage, as is 

 shown by the fact that a single blastomere of the 4-cell stage will — 

 provided that it contains a portion of the region of the grey crescent 

 — develop into a complete but miniature embryo, although in 

 normal development this blastomere would have furnished material 

 for only one-quarter of an embryo.^ Further, two embryos of the 



-cr 



Fig. 39 

 Diagram showing results of uniting pairs of Triton eggs in the 2-cell stage. The 

 future organiser-region (dorsal lip) is represented as a black crescent. Above, con- 

 dition when the first cleavage of both divides dorsal from ventral halves. Below, 

 condition when the first cleavage of both is median and divides right and left 

 halves. The result expected is a multiple monstrous form with three components. 

 (After Seidel, from Morgan, Experimental Embryology, Columbia University 

 Press, 1927, modified.) 



newt placed together cross-wise over one another at the 2-cell stage 

 can undergo development to form one single large embryo, provided 

 that the grey crescent regions of both are adjacent. (If these regions 

 are not adjacent, a double monster is produced'^ (%s. 39, 40).) 



The findings in the newt can be extended to other Urodela, and 

 in them it can be said that except for the determination of the region 

 of the grey crescent which will give rise to the dorsal lip of the 

 blastopore, or organiser, the cytoplasm of the ^^g is not unequally 

 distributed between the blastomeres up to the end of the 4-cell 

 stage inclusive. When animal and vegetative regions are divided, as 



^ Ruud, 1925. ^ Mangold, 1921A; Mangold and Seidel, 1927. 



