Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/174

126 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 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, n), 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 may be, the Vitellus divides into two segments, 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