374 Fertilization and Early Development of Pigeon's Egg 



In regard to the synchronousness of division in the sperm 

 nuclei, there appears to be a considerable difference. In one egg 

 of the two-celled stage, over one hundred sperm nuclei were present. 

 The accessory cleavage began at one side and here the nuclei had 

 nearly all passed into a resting stage. On the opposite side of the 

 disc, the nuclei were nearly all in some phase of division from spireme 

 to late anaphase. It did not seem that concentric zones could be distin- 

 guished, as Eiickert has found to be the case in the selachian, in which 

 the gradations in phase of division could be found in successive zones. 

 Eather in this egg there was a difference in phase on opposite sides, or 

 what may be called a polar difference. The evidence of this is seen also 

 in the beginning of the accessory cleavage, as shown in Fig. 40. 



The Yolk Nuclei of Later Cleavage Stages. 



With the advance of the cleavage nuclei, the sperm nuclei are driven 

 into the surrounding yolk. In a stage about fifteen hours after fertili- 

 zation, the sperm nuclei were found dividing amitotically. The inter- 

 vening stages have not been filled in. The identity of the yolk nuclei 

 at this stage with the earlier sperm nuclei is undoubted in the light of 

 the selachian egg, whose phenomena can be duplicated, at least as to chief 

 details, in the pigeon. 



Balfour has described the yolk nuclei in the chick as lying at the 

 margins of the blastoderm, and under the peripheral cells, but not 

 under the center. The nuclei are found very largely in nests or clusters, 

 the members of which are very unequal in size. Sometimes 6-8 nuclei 

 may be found thus clustered together (Figs. 39a, b, etc.). They are 

 surrounded by wide areas of protoplasm free from granules of yolk. 

 Besides the nuclear nests, many are found singly and these very fre- 

 quently at the margin of the blastoderm near the surface. These often 

 show a distinct difference in staining capacity, retaining the stain with 

 more tenacity than the underlying ones. This difference would seem 

 to be correlated with the environment, since these are found in the 

 coarse, deeply-staining, yellow yolk and the underlying ones in the white 

 yolk. There are also " giant " resting nuclei as large as the entire proto- 

 plasmic area which surrounds one of the nuclear clusters (Fig. 37b), 



Transitional stages from mitotic to amitotic division may be found 

 at this period. In some of the protoplasmic areas are found nests of 

 daughter nuclei not yet reconstructed (Fig. 38). The separate chromo- 

 somes or chromatin vesicles are distinct or partially fused together. In 

 some of the groups of chromatin vesicles approximately eight could be 

 counted, although the exact number could not be identified on account 



