16 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 
i d Cc b a 2 
Pia. L9.— Transverse section of the blastoderm of a pigeon’s egg about 
S} hours after fertilization (4.45 a.m.). (After Blount.) 
_1, Accessory cleavage. 2, Migrating sperm-nuclei. a,b, ec, 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 
Kia, 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 (6.380 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-nuclel 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, 
