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H. H. NEWMAN 



wherever half-sized and quarter-sized blastulae and gastrulae 

 occur they are to be regarded as the products of the physiological 

 isolation of the blastomeres of the two-cell or of the four-cell 

 stages. As has already been pointed out, some of the eggs in a 

 culture, after completing the first or the second cleavage, come 

 to rest for a period of several hours, and when cleavage is again 

 resumed, each blastomere behaves as though it were a complete 

 ovum and forms a complete half-sized or quarter-sized blastula. 



II 



Fig. 7 A good case of the early isolation of the blastomeres in the two-cell 

 stage and the subsequent independent cleavage of the two cells. 



Fig. 8 Twin blastulae within one vitelline membrane, probably derived 

 from a condition like that shown in figure 7. 



Fig. 9 A case of isolation of the blastomeres of the two-cell stage and its 

 consequences: a good four-cell stage and an irregular cleavage. 



Fig. 10 Twin blastulae, one of which is much more advanced than the other. 



Fig. 11 A case in which only one of the first two blastomeres has undergone 

 cleavage. This has formed a blastula with very little axiate organization. 



Fig. 12 Rare case of twin gastrulae, probably resulting from only partial 

 isolation of the cleavage products of the first two blastomeres. 



