270 KEPORT OF COMMISSIONER OF FISR AND FISHERIES. 

 EMBRYONIC DEVELOPMENT. 



The following- popular account of the early stages in the development 

 of the oyster is slightly modified from the description by Dr. W. K. 

 Brooks: 



The ovarian eggs are simply the cells of au organ of the body, the ovary, and they 

 difl'er from the ordinary cells ouly in being much larger and more distinct from each 

 other, and they have the power, when detached from the body, of growing and 

 dividing 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 wonderl'ul process may be 

 watched under the microscope, and owing xo the ease with which the eggs of the 

 oyster may be obtained this is a very good egg to study. 



About 15 miniites after the eggs are fertilized they will be found to be covered with 

 male cells, as shown in plate \u, tig. 1.* In about an hour the egg will be foiind to 

 have changed its shape and appearance. It is now nearly spherical, as shown in plate 

 VII, fig. 2, and the germinative vesicle is no longer visible. The male cells may or may 

 not still 1)e visible upon the outer surface. In a short time a little transparent point 

 makes its appearance on the surface of the egg and increases in size and soon forms 

 a little projecting transparent knob — the 2>olar (jlobule — which is shown in plate vii, 

 fig. 3, and in succeeding figures. 



Recent 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 disappear 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 ajipearance 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 (plate vii, fig. 4) 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. 



The upper portion of the egg soon divides up into smaller and smaller spherules, 

 until at the stage shown in plate vii, figs. 5, 6, and 7, we have a layer of small cells 

 wrapped around the greater part of the surface of a single large spherule, and the 

 series of figures shows that the latter is the spherule which is below in plate vii, fig. 4. 

 This spherule now divides up into a layer of cells, and at the same time the egg, or 

 rather the embryo, becomes flattened from above downward and assumes the shape 

 of a flat oval disk. Plate vii, figs. 10 and 9, are views of the upper and lower surface of 

 the embryo at about this time. In a sectional view, plate vii, fig. 11, it is seen to be 

 made of two layers of cells, an upper layer of small transparent cells, e c, which are 

 to form the outer wall of the body and which have been formed by the division of 

 the spherules which occupy the upper end of the egg in plate vii, fig. 6, and a lower 

 layer of much larger, more opaque cells, {/, which are to become the walls of the 

 stomach, and which have been formed by the division of the large spherule, a, of 

 plate VII, fig. 6. 



• This layer is seen in the section to be pushed in a little toward the upper layer, 

 BO that the lower surface of the disk-shaped embryo is not flat, but very slightly 

 concave. This concavity is destined to grow deeper until its edges almost meet, and 

 it is the rudimentary digestive cavity. A very short time after this stage has been 

 reached, and usually within from two to four hours after the eggs were fertilized, 

 the embryo undergoes a great change of shape and assumes the form which is shown 

 in three difterent views in plate vii, figs. 12, 13, 14, and 15. 



* References to figures in quoted portions of this paper do not correspond with the 

 originals, being altered to accord with their sequence in the present article. 



