44 



THE DEVELOPMENT OF THE CHICK 



everywhere in the hen's egg. In a section of a germinal disc, 

 showing the accessory cleavage (Fig. 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 



Fig. 17. — Photograph of an eight-celled pigeon ovum 

 (after Mary Blount). 2.45 a.m. Accessory cleavage 

 (ac. cl.) 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 disc. Orientation as in 

 Fig. 18. 



up to about the32-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 accesso-y 

 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 show^n 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. 



