dt THE DEVELOPMENT OF THE CHICK 
everywhere in the hen’s egg. In a section of a germinal disc, 
showing the accessory cleavage (lig. 20), it is seen that the 
peripheral boundary of the marginal cells cuts under the margin 
for a considerable distance. 
The accessory cleavage becomes manifest at the time of 
appearance of the first cleavage plane, and increases in amount 
Fic. 17. — Photograph of an eight-celled pigeon ovum 
(after Mary Blount). 2.45 a.m. Accessory cleavage 
(ac. el.) in the marginal zone bounding the segmented 
area. Vesicles, appearing black in the photograph, 
are seen on the surface of the yolk beyond the mar- 
ginal zone of the germinal dise. Orientation as in 
Fig. 18. 
up to about the 32-celled stage,and thereafter gradually decreases 
until it completely disappears (Figs. 18 B, C, and D). The 
peripheral boundaries of the marginal cells disappear pari passu, 
and, when the accessory cleavage is finally wiped out, the mar- 
ginal cells are everywhere continuous with the periblast, as in 
the hen’s egg (Figs. 18 B and C). In some eggs the accessory 
cleavage is much more extensive than in others; indeed, in some 
it appears to be entirely absent, but this is relatively rare. In 
the stage shown in Fig. 18 B, for instance, there is usually con- 
siderable accessory cleavage; but in this egg there is none. The 
variation is obviously due to variations in the number of super- 
numerary spermatozoa, such as may readily occur. 
