FISHERIES OF THE UNITED STATES. [22] 



ovarian eggs are simply tbe cells of an organ of the body, the ovary, 

 and tbey differ from the ordinary cells only in being much larger and 

 more distinct from each other; and they have the power, ^yhen detached 

 from the body, of growing and dividing ap into cells, which shall shape 

 themselves into a new organism like that from whose body the (^gg came. 

 Most of the steps in this wonderful process ma,y be watched under the 

 microscope, and, owing to the ease with which the eggs of the oyster 

 may be obtained, this 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 Fig. 51. In about an 

 hour the egg will be found to have changed its sha^ie and appearance. 

 It is now nearly spherical, as shown in Fig. 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 transparent point makes its 

 aijpearance on the surface of the egg, and increases in size, and soon 

 forms a little projecting transparent knob — the polar glohule — which is 

 shown in Fig. 3 and in succeeding figures. 



"Eecent investigations tend to show that while these changes are 

 taking place one of the male cells penetrates the protoplasm of the 

 egg and unites with the germinative vesicle, which does not disa])pear, 

 but divides into two parts, one of which is pushed out of the egg and 

 becomes the polar globule; while the other remains behind and becomes 

 the nucleus of the developing egg, but changes its appearance so that it 

 is no longer conspicuous. The egg now becomes pear-shaped, with the 

 polar globule at the broad end of the pear, and this end soon divides 

 into two parts, so that the egg (Fig. 6) is now made of one large mass 

 and two slightly smaller ones, with the polar globule between them. 



"The later history of the egg shows that at this early stage the egg 

 is not perfectly homogeneous, but that the protoplasm which is to give 

 rise to certain organs of the body has separated from that which is to 

 give rise to others. 



"If the egg at the stage shown in Fig. G were split in the plane of 

 the paper, we should have what is to become one-half of the body in one 

 one i)art and the other half in the other. The single spherule at the 

 small end of the pear is to give rise to the cells of the digestive tract of 

 the adult, and to those organs which are to be derived from it, while 

 the spherules at the small end are to form the cells of the outer wall of 

 tbe body and the organs which are derived from it, such as the gills, 

 the lips, and the mantle, and they are also to give rise to the shell. 

 The upper portion of the egg in this and succeeding figures is to be- 

 come the ventral surface of the adult oyster, and the surface which is 

 on the right side in Fig. G is to become the anterior end of the body 

 of the adult. The figure, therefore, shows the half of the egg which is 

 to become the left half of the body. The upper ])ortion of the egg soon 

 divides up into smaller and smaller spherules, until at the stage shown 

 in Figs. 2i, 25, and 2G w^e have a layer of small cells wrapped around 



