314 



THE SEGMENTATION OE THE OVUM. 



The Single Polar Globule. — As stated above, when only a single polar globule 

 is formed, it appears in the ovum in the ampulla of the tube. The nucleus moves 

 toward the surface, loses its membrane, and produces a polar spindle, which is 

 much smaller than that previously described. It lies at right angles to the axis 

 of the ovum and quite close to the surface. It contains twelve thick, achro- 

 matic fibers, which do not unite at the poles with one another, but end parallel, 

 so that the tip of the spindle is blunted. The chromosomes, when the membrane 

 first disappears, lie irregularly, but shortly after the formation of the spindle they 

 collect together to form an equatorial plate, somewhat as in figure 178. 

 They are irregular and of uneven size, twelve in number, or possibly the number 

 may vary somewhat. The chromosomes then divide transversely, the halves 

 move rapidly toward the ends of the spindle, which during this change passes into 

 the radial position (Fig. 179). The surface of the ovum or the apex of the 



Fig. 179.— Ovum of White Mouse, dividing to 

 Produce the Polar Globule. X S°° 

 diams. — (After J. Sobotta.) The elongated 

 male pro-nucleus lies in the inferior protuber- 

 ance of the ovum. 



'■■ -MX 





Fig. 1S0. — Ovum of White Mouse, Showing 

 the Metaphase of the Division Produc- 

 ing the First Polar Globule. X 'S 00 

 diams. — \ After J. Sobottu.) ■ 



spindle forms a protuberance, and the spindle moves partly into this protuberance. 

 Division of the achromatic fibers takes place, and there is formed a well-marked 

 cell plate (Fig. 180), and presently the polar globule becomes constricted off. 

 The cell plate appears with unusual distinctness. It is at about this stage that 

 the spermatozoon is found to have entered the ovum (Fig. 179), and to have 

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

 appears at the poles of the spindle, and no astral rays appear in the protoplasm, 

 although in many eggs these astral figures are extremely conspicuous. The nu- 

 clear elements in the ovum proper appear at first as a dense cluster of chromatin 

 granules. These 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 distinct membrane, and presently shows a network structure in its 



