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 



1 

 •> 





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



8| hours after fertilization (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 



Fig. 20. — Transverse section of the blastoderm of a pigeon's eg^ at the end 

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

 ization (6.30 A.M.). (After Blount.) 



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

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



as the hen's ovum of the same stage of development. It is doubt- 

 ful whether the absence of accessory cleavage in the hen's egg 

 should be taken as evidence that the fertilization is monospermic. 

 It may well be that supernumerary sperm-nuclei are present 

 without producing the appearance of accessory cleavage, owing, 

 perhaps, to a deeper situation in the periblast. This point 

 requires investigation. 



Another feature brought out by these photographs requires 

 emphasis. The periblast ring shows no definite outer margin, 



