19 
The body of the oyster, like that of all animals, except the 
Tery simplest, is made up of organs such as the heart, diges- 
tive organs, gills and reproductive organs, and these organs 
are at some period in the life of the oyster made up of mi- 
croscopic cells. The eggs shown in Figures 49 and 53, will 
. answer to illustrate the character of the cells which compose 
the body; each of these consists of a layer of protoplasm 
around a central nucleus, which, in the egg, is a large, cir- 
cular, transparent body known as the germinative vesicle. 
Each cell of the body is able to absorb food, to grow and to 
multiply by division, and thus to contribute to the growth 
• of the organ of which it forms a part. The ovarian eggs 
are simply the cells of an organ of the body, the ovary, 
and they differ from the ordinary cells only in being much 
larger and more distinct from each other; and they have the 
power, when detached frorii the body, of growing and di- 
viding up into cells, which shall shape themselves into a 
new organism like that from whose body the egg came. Most 
of the steps in this wonderful process may be watched under 
the microscope, and owing t# the ease with which the eggs 
of the oyster may be obtained, this is a very good egg 
to study. 
About fifteen minutes after the eggs are fertilized, they 
will be found to be covered with male cells, as shown in 
Figure 51. In about an hour the egg will be found to have 
'changed its shape and appearance. It is now nearly spheri- 
cal, as shown in Figure 1, and the germinative vesicle is 
no longer visible. The male cells may or may not still be 
visible upon the outer surface. In a short time a little trans- 
parent point makes its appearance on the surface of the egg, 
and increases in size, and soon forms a little projecting trans- 
parent knob — the polar globule — which is shown in Figure 
»3, and in succeeding figures. 
ltecent investigations tend to show that while these changes 
are taking place one of the male cells penetrates the proto- 
plasm of the egg and unites with the germinative vesicle, 
which does not disappear, but divides into two parts, one of 
