6 Mary Blount. 



Fig. 6) and were described by liim as follows: "The slightly oval 

 disc has a greater diameter of 3.5 mm. It is divided into two zones 

 quite clearly distinguished in opacity, the outer zone being due to 

 the abrupt thinning out of the fine granular matter of the disc." 

 Tliese two areas are the blastodisc and the periblast. In an egg 

 obtained at 11 p.m., about three hours after fertilization, the blasto- 

 derm was 2.96 mm. in transverse and 3.08 mm. in longitudinal 

 diameter. These measurements were taken on the living egg. Fig. 

 45 shows the surface view of such an egg. 



The central transverse section of another egg gives the diameter 

 of the blastodisc about 2.5 mm. This egg was taken from the 

 oviduct at 11.30 p.m. (three and one-half hours after fertilization). 

 In the sections I found the male and female pronuclei and a number 

 of supernumerary sperm nuclei. The latter had migrated peripherally 

 from their place of entrance and occupied a circle at the inner 

 margin of the periblast. Several nuclei were peripheral to those 

 at the inner margin of the periblast. They are probably not nuclei 

 of the original migration, but may be sisters to those just central 

 to them, if we may suppose that a nuclear division has taken place 

 since the migration into the periblast. Fig. 2 shows a central 

 transverse section of this egg, but the nuclei were in several successive 

 sections. See also a diagram traced from this figure, Fig. 3. 



The distribution of the sperm nuclei, their disappearance, and 

 later the distribution of the periblast nuclei are illustrated in seven 

 charts. The figure lettered A in each chart represents the surface 

 view of the egg. The orientation is the same for all the surface 

 views presented in this thesis. The anterior side of the blastoderm 

 is away from the observer, and the axis of the future embryo is 

 ill a diagonal position, as indicated in Fig. 1. These fig-ures (A of 

 the charts) were drawn from the living eggs in salt solution to the 

 same scale as nearly as possible in free hand work. \Vlien a dotted 

 circle occurs in A or B of any chart, it represents the apparent 

 peripheral limit of the marginal periblast. The supernumeraiy 

 sperm nuclei, and later the periblast nuclei, may migrate peripherally 

 to this distance. How much further the cytoplasm of the periblast 

 extends I do not know : for I cannot demonstrate the existence of 



