24 



CLEAVAGE AND THE GERM LAYERS 



and a cavity, the blastoccele, is enclosed by the cells. The embryo at this stage is 

 sometimes called a morula (mulberry). In subsequent cleavages, as development 

 proceeds, the size of the cells is diminished whOe the cavity enlarges (Fig. 15). 



^^ PB. 



Fig. is. — Cleavage of the egg of Amphioxus (after Hatschek). X200. 1. The egg before 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 slightlj' 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 bisected to- 

 show the cleavage cavity or blastoccele, B. 7. Later stage; the blastomeres have increased in number by 

 further division. 8. Blastula stage bisected to show the blastoccele, B. 



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

 The cleavage of the Amphioxus ovum is thus holoblastic, i. e., complete, and nearly 

 equal. 



