34 



SEGMENTATION OF THE FERTILIZED OVUM 



nearly spherical in form and about four hours old. The cleavage of the Amphioxus 

 ovum is thus complete and somewhat unequal. 



FIG. 14. Segmentation of the egg of Amphioxus, X 220 (after Hatschek). i. The egg before the 

 commencement of development; only one polar body, P.B, has been seen, but from analogy with other 

 animals it is probable that there are really two present. 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 bisected to show the segmentation cavity or blastocoel, B. 7. Later 

 stage: the blastomeres have increased in number by further division. 8. Blastula stage bisected to show 

 the blastocoel, B. 



The Ovum of the Frog. The ovum contains so much yolk that the nucleus and most 

 of the cytoplasm lies at the upper or animal pole. The first cleavage spindle lies in this cyto- 

 plasm. The first two cleavage planes are vertical and the four resulting cells are nearly equal 



