46 



THE DEVELOPMENT OF THE CHICK 



the accessory cleavage-cells and also in the unsegmented periblast 

 (Figs. 19 and 20), they decrease in number as the accessory 

 cleavage planes disappear, and when the latter are entirely lost 



2 



7" 



i5r 



1. 



-.2 





■iM 





Fig. 19. — Transverse section of the blastoderm of a pigeon's egg about 

 8| hours after fertihzation (4.45 a.m.). (After Blount.) 

 1, Accessory cleavage. 2, Migrating sperm-nuclei, a, b, c, d, Cells of 

 primary cleavage. 



the periblast is absolutely devoid of nuclei. Fragmentation of the 

 sperm-nuclei is a frequent accompaniment of their disappearance. 

 Thus the accessory cleavage is a secondary and transient 

 feature of the cleavage of the pigeon's egg due to polyspermy. 

 After it has passed, the ovum is in precisely the same condition 



P'iG. 20. — Transverse section of the l)lastoderm of a pigeon's egg at the end 

 of the period of multiplication of sperm-nuclei, about 10 hours after fertil- 

 ization (G..30 A.M.). (After Blount.) 



1, Accessory cleavage around the sperm -nuclei. 2, Marginal cells; sharply 

 separated from the sperm-nuclei. 3, Central cells. 4, Bperm-nuclei. 



as the hen's ovum of the same stage of development. In the hen's 

 egg Patterson has shown that there is a very limited and incon- 

 spicuous accessory cleavage (see Fig. 16 C) around the fewer 

 supernumerary sperm-nuclei that occur. But most of these nuclei 

 in the hen tend to pass into deeper portions of the disc and there 

 undergo complete fragmentation without producing superficial 

 furrows. 



Another feature brought out by these photographs requires 

 emphasis. The periblast ring shows no definite outer margin, 



