FERTILIZATION OF OVUM IN WHITE MICE. 



163 



be readily identified. The surface of the ovum or the apex of the spindle forms a 

 protuberance. Division of the achromatic fibers takes place, and there is formed a 

 well-marked cell-plate (Fig. 116), and presently the polar globule becomes con- 

 stricted off. The second body is smaller than the first, measuring from 7-1 2^ 

 in diameter, and in the majority of cases is the only one to be found inside the 

 zona after fertilization. Its twelve chromosomes soon form a resting membranate 

 nucleus. The cell-plate appears with unusual distinctness. It is at about this 

 stage that the spermatozoon is found to have entered the ovum (Fig. 117, B) and to 

 have formed there the male pro-nucleus. During all these stages no centrosome 

 appears at the poles of the spindle, but centrioles are said to have been observed 

 at the spindle apices. No astral rays appear in the protoplasm, although in many 



Pg.2 Pg.l 



FIG. 117. Two OVA OF WHITE MOUSE. A, WITH TWO POLAR GLOBULES. B, WITH THE SECOND POLAR GLOBULE 



ONLY. 



Pg. i, First polar globule. Pg. 2, Second polar globule. 6, Male pro-nucleus. 9 Female pro-nucleus. 



X 500 diams. (After J. Sobotta.) 



eggs these astral figures are extremely conspicuous. The female pro-nuclear ele- 

 ments appear at first as a dense cluster of chromatin granules (Fig. 117, J3?), and 

 fuse apparently into a compact mass, which grows rapidly in size, presumably by 

 the absorption of fluid from the yolk, and, as it enlarges, acquires a more distinct 

 outline, and presently shows a network structure in its interior (Fig. 117, A), with 

 irregular chromatin masses. It continues to grow more and more, and develops 

 at the same time a series of nucleoli more or less uniform in size. This stage 

 may be regarded as that of the completed female pro-nucleus. 



Fertilization occurs in the ampulla of the uterine tube about 6-10 hours after 

 the coitus. Unless it occurs the development of the second polar globule does not 

 take place. It is accomplished, normally, by the penetration of a single spermato- 

 zoon into the yolk. The tail of the spermatozoon usually enters the egg at least 

 in part. The head of the spermatozoon can be recognized at first by its shape 

 (Fig. 116, s). In- position it is typically more or less remote from the polar spindle. 

 While the second polar globule is forming the head assumes a rounded form, and 

 becomes the male pro-nucleus (Fig. 117, s). The group of twelve chromosomes left 



