370 Fertilization and Early Development of Pigeon's Egg 



phenomena and amoeboid movements occurring on the surface of the egg 

 along the circle whose plane sejDarates the two astral s^-stems. 



The Early Cleavages. — In Eigs. 40-45 are shown the 2-4-8-16 cell stages, 

 drawn from a surface view. The accessory cleavage is also shown, except 

 in the sixteen-cell stage. The first furrow crosses the disc along its 

 shorter diameter. It is slightly eccentric in position. In the four-cell 

 stage is seen the so-called " cross furrow " connecting the second furrows. 

 In the eight-cell stage, considerable variety exists in the position of the 

 furrows. A more regular type is shown in Fig. 43, and an irregular one 

 in Fig. 42. 



The more regular type shows meridional furrows at quite corresponding 

 positions in the four quadrants. In the sixteen-cell stage shown the 

 accessory cleavage is not represented (Fig. 45). The nuclei in the ana- 

 phase of division are shown as they appeared in a surface view in a whole 

 mount of the blastoderm, and give an idea of the relative size of nuclei 

 and blastomeres. 



In the sixteen-cell stage there is a clearly marked polarity of the 

 egg due to the small size of the blastomeres on one side. This asymmetry 

 of cleavage was pointed out by Kolliker in the case of the chick as pro- 

 ducing an evident polarity during the early cleavages, whose relation, 

 however, to the polarity of the embryo is undetermined. 



Development of the Accessory Nuclei. 



The accessory nuclei, whose appearance soon after the time of entrance 

 was described in connection with Fig. Gb, have been found in later stages, 

 varying considerably in number. In fertilization stages from twelve to 

 twenty-five have been counted. After division sets in among them their 

 number in some cases becomes very great, and no attempt has been made to 

 count them. The diagrams (Diagrams 1-6) show the number and dis- 

 tribution of these nuclei in a series of stages. The general fact is dis- 

 closed that they migrate away from the point of entrance and soon become 

 outside of the vicinity of the pronuclei. During the earlier stages of copu- 

 lation one or more of the accessory nuclei may remain in the vicinity 

 within the affected area of the germinal disc. But this is not true of the 

 later stages, as is indicated by the absence of accessory nuclei in 

 Figs. 14-16. 



In Fig. 13 a single pair are found at one side, near the affected area. 

 This pair are in close apposition, as if conjugating. Moreover, twenty- 

 five pairs of such nuclei are found in this egg together with some earlier 

 division stages. Some of the pairs are in apposition, but most of them 

 are a slight distance apart, some being in a stage very soon after division. 



