8 Mary Blount. 



in superficial positions in the marginal periblast, but those within 

 the circle bounding the blastodisc are in the region recognized as 

 the central periblast. They are therefore deep and are not confused 

 with the nuclei of primary cleavage. The following characters are 

 used in the charts : 



(^, male pronucleus. 9, female pronucleus. o, primary cleavage nucleus. 



6, periblast nucleus. +, sperm nucleus, bl., blastodisc. per., periblast. 



Chart I. 

 From a study of the sections, then, it becomes evident that before 

 the appearance of the first cleavage plane the supernumerary sperm 

 nuclei migrate into the periblast and occupy a circle which is later 

 characterized by accessory cleavage. This is illustrated in Chart I. 

 A central transverse section of the same egg is shown in Fig. 2. 



Chart II. 



Chart II A represents the surface view of an egg in the four- 

 celled stage. It was taken from the oviduct at 2 a.m., or about 

 six hours after fertilization. The blastodisc is incompletely divided 

 into four blastomeres which are continuous at their outer margins. 

 They are also continuous below with the yolk, see Chart II C and 

 Fig. 4. 



The blastodisc is surrounded by the periblast, the peripheral limit 

 of which as it appears in surface view, is indicated here by a dotted 

 circle. At the inner margin of the periblast there is an incomplete 

 zone of accessory cleavage. Where the accessory cleavage does not 

 appear, the blastomeres are continuous peripherally with the peri- 

 blast. This fact will be noted again in the description of later 

 stages. (Chart III A, Figs. 9, and 10 A and B.) 



In the sections fewer sperm nuclei were found than in the matura- 

 tion stage, which indicates that a varying number enter the egg 

 (Harper '04, p. 362). A number of sperm nuclei had migrated 

 in superficial positions to the outer margin of the zone which the 

 periblast presents in surface view and others were in the central 

 periblast. Although the central periblast is not at this stage sepa- 

 rated from the blastomeres, it may be identified with the finely 

 granular region below the blastodisc, and the thickness of the latter 



