CLEAVAGE 



mulberry. In subsequent cleavages, as development proceeds, the size 

 of the cells is diminished while the cavity enlarges (Fig. 14). The embryo 



FIG. 14. Cleavage of the egg of Amphioxus (after Hatschek). X 200. I. The egg be- 

 fore the commencement of development; only one polar body, P.B., is present, the other having 

 been lost during ovulation. 2. The ovum in the act of dividing, by a vertical cleft, into two 

 equal blastomeres. 3. Stage with four equal blastomeres. 4. Stage with eight blastomeres; 

 an upper tier of four slightly smaller ones and a lower tier of four slightly larger ones. 5. Stage 

 with sixteen blastomeres in two tiers, each of eight. 6. Stage with thirty-two blastomeres, in 

 four tiers, each of eight ; the embryo is represented bisec ted to show the cleavage cavity or blasto- 

 coele, B. 7. Later stage; the blastomeres have increased in number by further division. 8, 

 Blastula stage bisected to show the blastoccele, B. 



is now a blastula, nearly spherical in form and about four hours old. The 

 cleavage of the Amphioxus ovum is thus holoblastic, that is, complete, and 

 nearly equal. 



