NATURE AND CAUSES OF DIFFERENTIATION 



421 



determinate cleavage, essentially like that of the snail, the nuclei may 

 be shifted about by pressure without altering the end-result. Neither 

 can they lie in the form of the dividing mass as some authors have 

 assumed ; for in Crampton's experiments the half or quarter blasto- 

 mere does not retain the form of a half or quarter sphere, but rounds 



A 



B 



Fig. 191. — Double embryos of frog developed from eggs inverted when in the two-cell stage. 

 [O. SCHULTZE.] 



A. Twins with heads turned in opposite directions. B. Twins united back to back. C. Twins 

 united by their ventral sides. D. Double-headed tadpole. 



off to a spheroid like the ^gg. But if the limiting conditions lie 

 neither in the nucleus nor in the form of the mass, we must seek them 

 in the cytoplasm ; and if we find here factors by which the tendency 

 of the part to develop into a whole may be, as it were, hemmed in, we 

 shall reach a proximate explanation of the mosaic-Hke character of 

 cleavage shown in the forms under consideration, and the mosaic 



