898 



GENERATION. 



*$? 



little pedicle. There is then, usually, a second globule 

 formed just behind the first, in the same manner ; and 

 sometimes a third makes its appearance. As soon as 

 the globules are perfectly formed, they all become de- 

 tached from the vitellus, but remain adherent to each 

 other, gradually fusing to form a single, rounded, very 

 faintly granular mass ; and it is opposite this globule 

 that the first furrow of segmentation of the vitellus is 

 observed. The complete formation of the polar globules 

 and their fusion into one occupy three hours. It is prob- 

 able that the polar globule is formed in the mammalia in 

 the manner above indicated. Sometimes the polar glob- 

 ule is formed in ova that have not been fecundated. 



Vitelline Nucleus. A short time after the complete 

 formation of the polar globule, the germinal vesicle hav- 

 ing disappeared, the deformed vitellus resumes its original 

 rounded appearance and fills again the cavity of the vi- 

 telline membrane. At this time, the extreme periphery 

 of the vitellus becomes clearer, the granules collect in a 

 large zone around the centre, and, in the centre itself, a 

 clear, rounded body makes its appearance, which is called 

 the nucleus of the vitellus. This mass is viscid, amorphous, 

 without granules, and is entirely different from the germi- 

 nal vesicle, having no nucleus at first, a nucleolus, how- 

 ever, appearing in each of the numerous nuclei which re- 

 sult from its segmentation. The formation of the nucleus 

 of the vitellus is a positive evidence of fecundation. 

 It appears from fifteen to thirty hours after fecundation. 





FIG. 289. Segmentation of the vitel- 

 Iw. (Liegeois.) 



a, a, a, , a, spermatozoids. The four 

 upper figures represent the progres- 

 sive segmentation of the vitellus. 

 The lowest figure shows the cells of 

 the 'blastoderm. 



Segmentation of the Vitellus. Almost immediately 

 following the phenomena we have just described, the 

 vitellus begins to undergo the remarkable process of 

 segmentation, by which it is divided into numerous 

 small cells. This process may take place to a limited 

 extent in non-fecundated ova ; but in this case the 

 cells soon disappear, as the disintegration of the ovum 

 advances. The true segmentation of the vitellus, how- 

 ever, results in the formation of what are called the 

 blastodermic cells. As segmentation has been studied 

 in the inferior animals, there appears first a furrow in 

 the vitellus, at the site of the polar globule, and there 

 is then a furrow on the opposite side, both deepening 

 until the entire vitellus is divided into two globes. These 

 are at first spherical ; but they soon become flattened 

 upon each other into two hemispheres. There follows 

 then a similar division into four, another into eight, and 

 so on, until the entire vitellus is divided into numerous 

 cells, each with a* clear nucleus resulting from the seg- 

 mentation of the original nucleus of the vitellus. It is 

 probable that, at first, the cells of the vitellus have no 

 membrane ; but a membrane is soon formed, a nucleus 

 appears, and the cells are perfect. 



