164 



STUDY OF THE SEGMENTATION OF THE OVUM. 



in the ovum after the division of the polar spindle becomes the female pro-nucleus 

 (Fig. 117, 9). Both pro-nuclei now enlarge, the female most, and assume a nearly 

 spherical form, but have no membrane (Fig. 117, A). They approach one another, 

 drawing also toward the center of the ovum, until they come to lie side by side, 



FIG. 118. OVUM OF WHITE 

 MOUSE. BEGINNING OF THE 

 CONJUGATION OF THE PRO- 

 NUCLEI. X isoodiams. (After 

 Sobotta.) 



FIG. 119. OVUM OF WHITE 

 MOUSE. CONJUGATION OF THE 

 PRO-NUCLEI, AND FORMATION OF 

 THE SEGMENTATION SPINDLE. 

 X 1500 diams. (After Sobotta.) 



yet separated by a small space. The chromatin of the' two pro-nuclei forms dis- 

 tinct threads. Next there appears in the space between them a centrosome with a 

 few radiating lines around it (Fig. 118). From the centrosome arises, just how is 

 not clear, a spindle of achromatic threads (Fig. 119). The chromatin of each pro- 



FIG. 1 20. OVUM OF WHITE 

 MOUSE. FIRST SEGMENTATION 

 SPINDLE WITH THE CHROMO- 

 SOMES OF THE PRO-NUCLEI STILL 

 FORMING Two DISTINCT 

 GROUPS. X 1500 diams. 

 (After Sobotta.) 



FIG. 121. OVUM OF WHITE MOUSE. FIRST SEG- 

 MENTATION SPINDLE WITH EQUATORIAL PLATE 

 OF CHROMOSOMES. X 1500 diams. (After 

 Sobotta.) 



nucleus now forms a group of well-defined, elongated, somewhat crooked chromo- 

 somes. The two groups of chromosomes are quite distinct, and are separated from 

 one another by the intervening spindle (Fig. 119). The spindle continues to grow, 

 and the chromosomes of the male pro-nucleus on the one side and the female pro- 



