SEGMENTATION OF THE VITELLU3. 357 



cess may take place to a limited extent in non-fecundated 

 ova ; but in this case the cells soon disappear, as the disinte- 

 gration of the ovum advances. The true segmentation of the 

 vitellus, however, 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 is 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 result- 

 ing from the segmentation 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 nu- 

 cleolus appears and the cells are perfect. 



Most of the phenomena of segmentation have been ob- 

 served in the lower orders of animals ; but there can be no 

 doubt that analogous processes take place in the human ovum. 

 In the rabbit, Weil observed, forty-five and a half hours after 

 copulation, an ovum, with sixteen segmentations, situated in 

 the lower third of the Fallopian tube. Ninety-four hours 

 after copulation, he observed an ovum, with a delicate mosaic 

 appearance, presenting a small, rounded eminence on its sur-. 

 face. 1 



It is impossible to say how long the process of segmenta- 

 tion continues in the human ovum. It is stated that it is 

 completed in rabbits in a few days, and in dogs, that it occu- 

 pies more than eight days." When the cells of the blastoderm 

 are completely formed, they present a polygonal appearance 

 as they are pressed against the vitelline membrane, their in- 

 ner surface being rounded. The ovum then contains, within 

 the external layer of cells, a small quantity of liquid. It is 



1 WEIL, loc. cit., S. 26. 



2 HERMANN, Grundiss der Physiologic, Berlin, 1870, S. 469. 



