18 Fertilization and Early Development of Pigeon's Egg 



The nuclei and first furrow have been found at a stage when the nuclei 

 are very much larger. They had moved apart relatively little compared 

 with the former stage, while they had greatly increased in size. If the 

 small size at the former stage indicated that little time had elapsed since 

 division, then their movements at first must have been more rapid. Per- 

 haps the cytoplasmic constriction would be the cause of the early, rapid 

 separation of the nuclei. Whatever may be the link connecting nuclear 

 with cell division, it would seem that the constriction of the cytoplasm 

 must play a part in the separation of the nuclei. At this stage, the 

 completion of the first division, there is no differentiated area about the 

 nuclei recognizable. Apparently the amoeboid changes cease during the 

 resting stage of the nuclei. 



The Second Division. — In Fig. 16 is shown the beginning of the second 

 division. The first furrow is longer. The nuclei are about equidistant 

 from it. The spindles are formed and are in the prophase, approaching 

 the equatorial plate stage. They lie in small areas free from granules. 

 The differentiation in the cytoplasm is like that surrounding the first 

 cleavage spindle, except for greater complexity. There is a clearly 

 marked polarity. One blastomere is more granular, the other more 

 hyaloplasmic. In the latter there is a complex affected area surrounded 

 by homogeneous protoplasm. The hyaline border of the active area is 

 even more distinct than at the first division. But this blastomere mu&t 

 apparently be identified with the more hyaloplasmic pole at the first 

 division. The hyaline border shows a sinuous contour. The whole area 

 is elongated, as before, in the plane of the next nuclear division. The 

 prominences or outpushings are more complex. They correspond some- 

 what at the two poles, but are more developed at one pole (the left). 

 There is an evident beginning of constriction, and division at this point 

 would separate again a more hyaloplasmic blastomere from a more granu- 

 lar one. In the other blastomere the affected area has an even contour. 

 One other egg was obtained at this stage and shows the same general fea- 

 tures, but with minor differences in the apparent amoeboid changes. It 

 would hardly be expected that amoeboid movements of this character 

 would give rise to identical appearances in different eggs. The observa- 

 tion of Whitman, 87, on cytokinetic phenomena in general may be quoted 

 in this connection: "They are diversiform in the. extreme, rarely pre- 

 senting regular form series, and thus stand in marked contrast with 

 nuclear metamorphoses, which everywhere, both in plant and animal 

 cells, exhibit a most remarkable uniformity." 



Comparison of Clmvage and Maturation Divisions. —The appearances 

 here described in connection with cleavage may be compared with the 



