24O MARY BLOUNT. 



5 . The Disappearance of the Sperm Nuclei. — Fig. 6 represents a 

 longitudinal section through about the center of the blastoderm 

 of an egg taken from the oviduct at 7:00 A. M., or eleven hours 

 after the approximate time of fertilization. There were in this egg 

 a few remaining sperm nuclei, and where they occurred, they 

 were separated from the marginal cells by cleavage planes simi- 

 lar to those on the right of Fig. 5. In sections where the sperm 

 nuclei did not appear, the marginal cells were open to the peri- 

 blast, as on the left of Fig. 6. On the right of this figure (which 

 is the posterior side of the blastoderm) the marginal cell is partly 

 closed in, and in a few sections beyond this, it was entirely 

 closed, being separated from a cell of accessory cleavage. In 

 surface view, also, the marginal cells were open peripherally ex- 

 cept where the accessory cleavage occurred. 



In another egg taken from the bird at 7:00 A. M. (eleven 

 hours from fertilization) every marginal cell as seen in surf ace view 

 was open peripherally, and in sections, the margin was like that 

 at the left of Fig. 6. Not one nucleus was found outside the cells 

 of primary cleavage. 



Other eggs of about this period show accessory cleavage on 

 the wane and conditions in sections like those in Fig. 6. In the 

 egg represented in Fig. 5, the sperm nuclei were fragmenting. 

 They disappear between ten and twelve hours after fertilization. 



Fig. 7 is a photograph of an egg eleven hours from fertiliza- 

 tion (7:10 A. M.). Here, the central, marginal and periblastic 

 regions are clearly expressed. This is probably a stage after the 

 disappearance of the sperm nuclei, and nearly all of the marginal 

 cells are open peripherally. At the posterior side there are sug- 

 gestions of accessory cleavage. These small cells are probably 

 mere bud-like projections from the periblast, and not due to the 

 presence of sperm nuclei. It is impossible to decide this point 

 from surface view, but sections would show the relations between 

 the marginal cells and the periblast, and therefore demonstrate 

 whether the nuclei of these small cells were derived from super- 

 numerary sperms or from the cleavage nucleus. 



6. The Periblast. — Previous paragraphs have anticipated the dis- 

 cussion in this. Any mention of the periblast refers the student 

 of vertebrate embryology to the work of Agassiz and Whitman 



