122 J. Thomas Patterson. 



The more important comparisons are as follows: 



1. The process of fertilization (that is, the entrance of the sperm) 

 in each egg occurs immediately after ovulation, when the egg is in the 

 region of the infundibulum. 



2. At the time of fertilization in the pigeon's egg, from twelve to 

 twentj-five supernumerary sj^erm nuclei enter the egg. In the hen's 

 egg only five or six such nuclei are found (except in one case where 

 twenty-four were present). 



3. Upon their entrance into the egg these sperm nuclei, in each 

 egg, migrate toward the periphery of the disc. In the pigeon's egg 

 the nuclei, on reaching the margin, become active, divide and give 

 rise to an accessory cleavage, which disappears between ten and 

 twelve hours after fertilization. In the hen's egg some of the super- 

 numerary nuclei pass down into the deeper portions of the disc and 

 there undergo complete fragmentation ; others may succeed in reach- 

 ing the margin, and there give rise to a rudimentary accessory 

 cleavage, which disappears shortly after the eight-celled stage, or 

 between four and five hours after fertilization. 



4. In the pigeon's egg the marginal cells become closed and remain 

 so throughout the period occupied by the accessory cleavage. In 

 the hen's egg the marginal cells always remain open to the periblast 

 both below and peripherally. This would seem to indicate that the 

 condition of a closed marginal cell in the pigeon's egg is to be cor- 

 related with the presence of a large number of accessory cleavages. 

 Perhaps it is for the purpose of cutting off some influence emanating 

 from the accessory sperm nuclei. 



5. In neither egg does the direction of the first cleavage plane, or 

 the eccentricity of cleavage, if present, seem to bear any constant rela- 

 tion to the axis of the future embryo. 



6. Immediately after the disappearance of the accessory cleavages 

 and their accompanying nuclei in the pigeon's egg the marginal cells 

 open to the periblast, and their nuclei divide and some of the daughter 

 nuclei migrate into the periblast and "organize" it. In the hen's 

 egg there is a period of from two to three hours after the disappear- 

 ance of the accessory sperm nuclei during which the periblast is 

 void of nuclei of any kind. 



