6 MARGARET MORRIS 



After the formation of the second polar body, the chromo- 

 somes that have remained in the egg form a large female pro- 

 nucleus. The sperm nucleus has enlarged, in the meantime, 

 and the two nuclei unite (figs. 10 and 11). No attempt has 

 been made to trace the history of the aster which appears before 

 the tw^o nuclei fuse. 



The first cleavage spindle does not lie in the center of the 

 egg, as the first division is an unequal one. It is about the 

 size of the first polar spindle, but the chromosomes are verj'^ 

 different in form as well as in number from those of the matura- 

 tion divisions. In the cleavage spindles they are long, thin rods, 

 or threads, with sUght terminal swellings. They are so much 

 bent and intertwined in the equatorial plate that an accurate 

 count is impossible, but the number is presumably thirty-six. 

 Figure 13 shows the anaphase of the first cleavage, in which the 

 chromosomes are small rods. Throughout the early cleavage 

 the chromosomes have, in the metaphase, the form of long 

 threads; but in the later development a gradual change of shape 

 is seen. Figure 15 shows chromosomes from a middle cleavage 

 stage, and figure 16 is from an egg fixed nine hours after ferti- 

 lization. Here the chromosomes have been reduced to the short 

 rods which Jordan says are characteristic of Cumingia, and 

 which are considerably smaller than the chromosomes of the 

 fourth cleavage, for instance, shown in figure 14. 



The normal cleavage pattern of Cumingia has been described 

 by Browne ('10) in her study of the effect of pressure. It is 

 illustrated here for comparison with the cleavage of partheno- 

 genetic eggs in the text-figure 1 (surface views) and figures 17 

 to 28 (sections). The first cleavage plane passes through the 

 polar bodies and divides the egg into two unequal blastomeres 

 (fig. 17, text-fig. I, 3 and 4). Of these the larger one (lettered 

 CD in the drawings) is usually the first to divide. The spindle 

 forms in the middle of the cell, but moves to an eccentric posi- 

 tion, with the outer end slanted towards the cell AB, while still 

 in the metaphase (figs. 19 and 20). The result of this division 

 is a 3-cell stage in which the cells are all unequal in size, C being 

 equal to about half of AB (figs. 23 and 24), text-fig. I, 6 and 7). 



