CLEAVAGE 



237 



one of the first two cleavage planes, usually the first, becomes 

 the plane of a bilateral symmetry which may remain quite pro- 

 nounced for some time, and indeed corresponds with the median 

 plane of the bilaterally symmetrical adult. The bilateral type 



FIG. 113. Meroblastic cleavage in the squid, Loligo pealii. A, B. Egg viewed 

 obliquely, showing animal pole. X 45. After Watase. C, D. Surface views of 

 animal pole, more highly magnified, to show bilateral arrangement of blasto- 

 meres. From Wilson, "Cell," after WatasS. A. Four-cell stage. B. About 

 sixty-cells. Cells at animal pole very small, lowermost cells incomplete, cell 

 walls extending down toward the uncleaved lower pole. C. Eight-cell stage. 

 D. The fifth cleavage (sixteen to thirty-two cells), a-p, marks the plane of 

 the first cleavage and the median plane of the organism; l-r, marks the second 

 cleavage, and the transverse plane of the organism. 



of cleavage is found in the Cephalopods (Fig. 113), a few 

 Rotifers and Nematodes, in Amphioxus and the Ascidians, and 

 perhaps in most of the Craniates, but in these last forms varia- 

 tions are more frequent, especially in those forms with discoid 

 cleavage. 



