EMBRYONIC DEVELOPMENT. 109 



The segmentation or cleavage of the ovum results in 

 the formation of a spherical blastula, closed at all points, 



B c 



Pis- 57- — Three stages in the segmentation of the ovum, according to the 

 bilateral type. From the lower pole. (After E, B. WILSON.) 



A. Eight-cell stage. A, B, C, D. The four macromeres, above which are seen 

 portions of the four micromeres. 



/-/. Plane of first cleavage, with respect to which the cells divide in such a way 

 as to become arranged in a bilaterally symmetrical manner. 

 //-//. Plane of second cleavage. 



B. Transition to the si.xteen-cell stage. 



C. Sixtecn-cell stage. The line in each cell indicates the direction in which the 

 next division of the cell would take place. 



and consisting of some 256 cells surrounding a spacious 

 cavity, the blastocoel. 



The stages of development lead- 

 ing up to the blastula are known 

 as the segmentation stages. At 

 their completion, although, of 

 course, cell-division continues to 

 take place actively, yet other pro- 

 cesses supervene which render the Fig. 58. — Eight-ceii stage 



... ... from the upper pole, illlustrat- 



mere division of the individual cells mg the spiral type of cleavage. 



of minor morphological importance. ^^'*'^'' ^' ^' 'w^son.) 



Gastrulation. 



The next phase of the development is known as the 

 gastrulation of the embryo. The cells forming the lower 

 or vegetative side of the blastula remain, throughout the 

 segmentation stages, somewhat larger than the rest of the 



