THEORIES OF INHERITANCE AND DEVELOPMEXT 



the development of the blastomere is from the beginning hemmed 

 in by the cytoplasmic conditions, and its powers are correspondingly 

 limited. In such cases the cleavage may exhibit more or less of 

 a mosaic-like character, and the theory of cytoplasmic localization 

 acquires a real meaning and value. 



That we are here approaching the true explanation is indicated by 



D 



Fig. 140. Double embryos of frog developed from eggs inverted when in the 2-cell stage. 

 [O. SCHULTZE.j 



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. 



certain very remarkable and interesting experiments on the frog's 

 egg which prove that each of the first two blastomeres may give rise 

 either to a half-embryo or to a whole embryo of half size, according 

 to circumstances, and which indicate, furthermore, that these circum- 

 stances lie in a measure in the arrangement of the cytoplasmic 

 materials. This most important result, which we owe especially to 



