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COMPARATIVE ANATOMY 



lines are continued downwards into the yolk blastomeres. 

 But these, being so largely composed of inert deutoplasm, 

 divide slowly, and their division is completed some time 

 after it has been effected in the upper blastomeres. The 

 next divisions may be described as equatorial, and affect 

 all the sixteen blastomeres, dividing each of them into two, 

 as is shown in fig. 26, C. The embryo now consists of thirty- 



Fig. 27. 



A, vertical section through a segmenting ovum at about the stage 

 represented in fig. 26 D. B, C, and jO, similar sections through 

 later stages. Bit segmentation cavity or blastoccele, d^, blastopore. 

 (After Morgan.) 



two cells, and the succeeding divisions become irregular, and 

 are no longer synchronous. The upper pigmented cells divide 

 much faster than the lower yolk-cells, and the final result of 

 the first phase of development (usually called the segmentation- 

 phase) is a vesicle containing a rather flattened and eccentric 

 cavity, whose roof is composed of two layers of small pig- 



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