234 



MARY BLOUNT. 



section in Fig. 3. The dotted circle, Fig. 2, represents the dis- 

 tance to which the sperm nuclei may migrate peripherally (it is 

 also the peripheral limit of the periblast nuclei, to be explained 

 later), but the accessory cleavage, with a few exceptions, is con- 



FiG. 2. Sketch of surface view of a pigeon egg about eight and three fourths 

 hours after fertilization, 4:45 A. M. a, b, c, d, cells of primary cleavage which are 

 shown in section in Fig. 3. xy, the plane of the section in Fig. 3. 1. Accessory 

 cleavage. 2. Periblast. 



fined to the zone just outside the blastomeres of the primary area. 

 A migrating sperm nucleus is shown at the extreme right of Fig. 

 3. Another sperm-nucleus has migrated under the large blasto- 

 mere a. It is in the central periblast, which will be explained 

 later. A study of the whole series of sections showed a number 

 of nuclei in this position, forming a submarginal circle. They 

 migrate later as far centrally as the margins of the nucleus of 

 Pander, but were never found under the very center of the 

 blastoderm. 



Fig. 3 is of a section taken through the plane xy of Fig. 2. 



c b a 2 



: li-w 





Fig. 3. Transverse section of the pigeon egg whose surface view is shown in 

 Fig. 2. . a, b, c, d, cells of primary cleavage. I. Accessory cleavage. 2. Migrating 

 SDerm nuclei. 



