SEGMENTATION 



127 



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. />', C, and I), similar sections through 

 later stages. Bl, segmentation cavity or blastoccele, bp^ blastopore. 

 (After Morgan.) 



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

 1 1 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 



