Development 45 



power that its cell descendants retain. The man- 

 ner of its division or segmentation in the American 

 oyster will illustrate the process in the group of the bi- 

 valves, and, briefly following the description of Pro- 

 fessor Brooks, is as follows : 



About an hour after the male cell has entered the 

 ovum, the latter becomes somewhat enlarged at one end 

 that to which the pole cells are attached. The nucleus 

 of the ovum divides, the two nuclei separate, and a con- 

 striction of the body of the cell separates it into two cells. 

 The cell to which the pole cells are attached soon di- 

 vides, and a stage represented by II in Figure 1 1 appears. 

 Here are shown one large and two slightly smaller cells. 

 Preceding every division, there is a division of the 

 nucleus, so that every cell always contains a nuclear 

 body. 



Even thus early in development, it is possible to de- 

 termine a difference in the fate of these cells. From the 

 single larger one, will arise the digestive tract, and 

 from the two smaller, will be formed the outer wall of 

 the body. 



In the course of a few minutes, if the temperature is 

 not below 70 Fahrenheit, the two smaller cells divide, 

 forming four, each of which is about half the size of the 

 cell from which it is derived. Again after a pause, there 

 is another division of the small cells. One more division 

 gives us the segmenting egg represented by VI, in which 

 one large cell is partially covered by a cap of several 

 smaller ones. 



The multiplication of the smaller cells continues, and 

 they spread still farther over the surface of the larger 

 one. The latter finally divides, forming an inner layer. 

 The relative positions of these parts is illustrated by VII, 



