126 EVOLUTION OF LIFE. 
the present to the animal kingdom, let us examine the cell, 
or the egg, of a mammal,—that of a rabbit (Fig. 160), 
guinea-pig, or man, for example. The egg of a mammal, 
about the 135 of an inch in diameter, when magnified, is 
seen to consist of a cell-wall or Vitelline membrane (Fig. 
160), inclosing cell-contents, or the Vitellus, in which is 
found the nucleus (Fig. 160, z), or Germinal vesicle, with 
its nucleolus, or Germinal spot. Let us observe what takes 
place, supposing the conditions to be favorable to the 
development of the egg. According to some observers, 
the Germinal vesicle and spot disappear; equally good 
observers, however, state that the Germinal vesicle and 
spot divide into two. While there is some doubt as to the 
disappearance of the Germinal vesicle and spot, all observers 
agree that the Vitellus, or cell-contents, divide into two 
segments (Fig. 161), and that each segment has its nucleus 
and nucleolus, As the segments are the halves of the 
Vitellus, probably the nuclei and nucleoli are formed 
through the division of the Germinal vesicle and spot. 
However this maybe; the Vitellus divides into two seg- 
ments, each segment having a nucleus with its nucleolus. 
These two segments subdivide into four balls (Fig. 162), 
the four into eight (Fig. 163), the eight into sixteen (Fig. 
164), and so on. Through this process of cell-division, or 
segmentation, as it is called, the Vitellus is divided into a 
number of little balls, and assumes the shape of a mulberry. 
Finally, the superficial balls of the mulberry are transformed 
into cells, and so arrange themselves as to present the 
appearance of a mosaic pavement (Fig. 165); as the deeper 
balls become cells, they pass to the surface and increase 
the thickness of this mosaic-like membrane. In this 
manner the Vitellus is converted into a vesicle; within this 
vesicle there shortly appears a second vesicle; these two 
vesicles are usually called the Germinal layers, or the 
External and Internal blastodermic membranes. If the 
