^T: 



18 



INTEODUCTION. 





vn 



FtG. 2.— Segmentation of the egg of Amphioxus. x 220. (After Hatschek.) 



I, the egg before the commencement of development : only one polar boily, PB, liJis 

 beeu aeen, but from analogy with other animals it Is probable tliat there are really two 

 present. II, the ovum in the act of dividing, by a vertical cleft, into two equal blasto- 

 meres. Ill, stage with four equal blastomeres. IV, stage with eiglit blastomeres ; an 

 upper tier of four slightly smaller ones, and a lower tier of four slightly larger ones. 

 V", stage with fiixteeu blastomeres, in two tiers, each of eight. "VI, stage witli thirty- 

 two blastomeres, in four tiers, each of eight : the embryo is represented bisected, to show 

 the segmentation cavity or blastocrel, B. VII, later stage ; the blastomeres Ivave in- 

 creaseil in number by furtlier division. VIII, blastula stage ; bisected to show 

 the blastoccel, B. 



