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THEORIES OE INHERITANCE AND DEVEIOPMENT 



meres of sea-urchin eggs separated by shaking to pieces the two- 

 cell and four-cell stages. Blastomeres thus isolated segment as if 

 still forming part of an entire larva, and give rise to a half- (or quar- 

 ter-) blastula (Fig. 132). The opening soon closes, however, to form a 



Fig. 134. — Dwarf and double embryos of Amphioxus. 



A. Isolated blastomere of the 2-cell stage segmenting like an entire egg (cf. Fig, 123, Z)). 

 B. Twin gastrulas from a single egg. C. Double cleavage resulting from the partial separation, 

 by shaking, of the blastomeres of the 2-cell stage. D. E. F. Double gastrulas arising from such 

 forms as the last. 



small complete blastula, and the resulting gastrula and Pluteus larva 

 is a perfectly formed dwarf of only half (or quarter) the normal size. 

 Incompletely separated blastomeres gave rise to double embryos like 

 the Siamese twins. Shortly afterwards the writer obtained similar 

 result in the case of AnipJiioxus, but here the isolated blastomere seg- 



