106 



PART I. GENERAL ACCOUNT 



Fig. 73. Cleavage in Siboglimim caidleryi: Л-4 -cell stage, from the flattened side of the embryo; 

 В - stage of five blastomeres, from the flattened side of the embryo; C- the same from the convex 

 side. (After Ivanov, 1957b.) 



by side (Fig.73y4). The nuclei remain on the concave side of the egg at the time 

 of the first two divisions and now take up a position at the surface of the egg. 

 Cleavage now ceases to be synchronous. The big anterior blastomere 

 divides first into a large anterior and a smaller lateral cell, resulting in a 

 5-cell stage (Fig. 73i?, C). The further progress of cleavage remains unknown. 

 By the next known stage cleavage has already produced 80-85 blastomeres. 

 Differences in cell size are particularly noticeable. Very large cells are 

 situated at the hind end of the embryo and on the concave side, where 



their arrangement shows a regular 

 bilateral symmetry. On the concave 

 surface of the embryo, moreover, 

 a pair of very large eye-catching 

 cells lie near the anterior end. On the 

 convex side the size of the blasto- 

 meres increases gradually from small 

 at the anterior pole to large at the 

 posterior end of the embryo (Fig. 74). 

 In a further stage, consisting of 

 130-140 blastomeres, the same gen- 

 eral arrangements of the cells is 

 retained. On the convex side of the 

 embryo, however, the transition be- 



rig. 74. Lmbryo 01 biboglinum caidleryi at J ' ' 



the 80-85 cell stage: A - from the flattened tween small and large blastomeres is 



side of the embryo; 2?- from the convex side. more a b rU pt, taking ОП the nature of a 



ma - anterior pair of large blastomeres. f ; " 



(After Ivanov, 1957b.) boundary (Fig. lb A). On the concave 



ma. 



