800 GENERATION. 



studied by Robin in some of the lower forms of animals. The following are 

 the phenomena observed in the ova of the nephelis octoculata : 



Five hours after the entrance of the spermatozoids, a little elevation ap- 

 pears at one point in the vitellus. This is the beginning of the polar globule. 

 It increases in size gradually, and becomes constricted at its base, until it is 

 attached to the vitellus by a 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 detached from the vitellus, but remain adherent to each 

 other, gradually fusing to form a single, rounded, very faintly granular mass. 

 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 probable 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 having disappeared, the deformed vitellus 

 resumes its original, rounded appearance and fills again the cavity of the 

 vitelline membrane. At this time the extreme periphery of the vitellus be- 

 comes 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 germinal vesicle, having no nucleus at first, 

 a nucleus and a nucleolus, however, appearing in each of the many cells 

 which result from its segmentation. The formation of the nucleus of the 

 vitellus is positive evidence of fecundation. It appears fifteen to thirty 

 hours after penetration of the spermatozoids. 



Segmentation of the Vitellus. Almost immediately following the phe- 

 nomena just described, the vitellus begins to undergo the process of segmen- 

 tation, by which it is divided into a large number of small cells. This process 

 may take place to a limited extent in non-fecundated ova ; but in these in- 

 stances the cells soon disappear, as the disintegration of the ovum advances. 

 The true segmentation of the vitellus, however, results in the formation of 



FIG. 293. Segmentation of the vitellus (Haeckel). 



A, the vitellus divided into two cells : B, the two cells divided into four ; C, the four cells divided into 



eight ; D, the blastodermic cells. 



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 



