DEVELOPMENT PRIOR TO LAYING 45 
The question arises whether the disappearance of the cell- 
walls around the sperm-nuclei is caused by degeneration of the 
latter, or is simply a later syncytial condition in the periblast in 
Cc D 
Fic. 18. — Photographs of the cleavage of the pigeon’s ovum (after Mary 
Blount). The figures are so arranged that the axis of the shell is across 
fan) t 
the page with the large end to the left. The future axis of the embryo 
is therefore inclined 45° to the margin of the page with the anterior end 
to the right above. 
A. A very regular sixteen-celled stage; accessory cleavage well shown; 
though not well focused on the lower margin. 3.45 A.M. 
B. Approximate thirty-two celled stage. There is no accessory cleavage 
in this ease. The formation of the central from the marginal cells may be 
readily observed in this figure. 5.15 A.M. 
C. Later stage of cleavage. 7.10 A.M. 
D. Cleavage at 9.30 a.m. The marginal cells are now becoming separated 
peripherally from the periblast which has received its nuclei from them. 
which the sperm-nuclei are embedded. There can be little doubt 
that the former alternative is correct. While in the stages of 
the accessory cleavage, sperm-nuclei are readily found both in 
