DEVELOPMENT PRIOR TO LAYIXG 



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 



B 



C D 



Fig. 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 

 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 thirtv-two celled stage. There is no accessory cleavage 

 in this case. The formation of the central from the margmal cells may be 

 readily observed in this figure. 5.15 a.m. 



C. Later stage of cleavage. 7.10 a.m. , 



D. Cleavaiie at 9.80 a.m. The marjrinal cells are now becommg 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 m 



