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 



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 egg 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, 



