SEGMENTATION 



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tion has divided the ovum into two equal halves, or blastomeres. 

 A pause follows, and the ovum divides again, the plane of 

 the second division being at right angles to the first. Thus 

 four blastomeres are formed, each consisting of a smaller 

 upper pigmented protoplasmic part, and a lower colourless part 

 filled with deutoplasm. The third segmentation is described 

 as equatorial, but it does not pass through the equator of 



Fig. 28. 



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

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

 later stages. />/, segmentation cavity or blastocoele, bp, blastopore. 

 (After Morgan.) 



the spherical ovum, but is placed nearer the pigmented pole. 

 It nearly completely cuts off four pigmented upper blastomeres 

 from four lower heavily-yolked blastomeres. Hitherto the 

 planes of division have passed right through the ovum, 

 giving rise to an eight-celled stage, and the next divi- 

 sion is described as meridional. Each upper pigmented 



