IMPREGNATION OF THE OVUM. 53 



The chromatin of each pro-nucleus now forms a group of well-defined, elongated, 

 somewhat crooked chromosomes. The two groups of chromosomes are quite 

 distinct, and are separated from one another by the intervening spindle. The 

 spindle continues to grow, and the chromosomes of the male pro-nucleus on the 

 one side, and the female pro-nucleus on the other, attach themselves to the 

 equatorial region of the spindle (Fig. 7). The spindle continues to grow; the 

 chromosomes become V-shaped and arrange themselves as the so-called equa- 

 torial plate, in which the chromosomes of the two pro-nuclei can no longer be 

 distinguished from one another. At each end of the spindle is a distinct centro- 

 somewith a very faint, small astral radiation in the neighboring protoplasm. This 

 spindle is the beginning of the division of what we may call the segmentation 



n 



Fie. 5. — Ovum of White Mouse. 

 Beginning of the Conjuga- 

 tion of the Pro-nuclei. X 

 1500 diams. — {After Sobotta.) 



Fig. 6. — Ovum of White Mouse. 

 Conjugation of the Pro- 

 nuclei, and Formation of 

 the Segmentation Spindle. 

 X 'SCO diams. — {After Sobotta.) 



Fig 7. — Ovum of White Mouse. 

 First Segmentation Spindle 

 with the Chromosomes of 

 the Pro-nuclei still form- 

 ing Two Distinct Groui>s. 

 X 1500 diams. — {After Sobotta. \ 



nucleus. In the mouse the two pro-nuclei do not actually fuse into a single 

 nucleus before the formation of the spindle, which initiates the first division of 

 the fertilized ovum, so that, strictly speaking, there is no fusion of the pro-nuclei 

 to make a segmentation nucleus. There is, nevertheless, a true fusion of the 

 pro-nuclei accomplished, although it is somewhat masked by the early com- 

 mencement of the first segmentation spindle, which develops at the same time 

 that the fusion of the pro-nuclei is being completed. Whether the processes as 

 described in the mouse are typical for mammals we do not know, the white mouse 

 being the only species in which the process has been followed in detail. The 

 student will find some additional information in the practical part. 



It is now believed that the pro-nuclei never unite to form a distinct mem- 



