246 A Sketch of Comparative Embryology. [ April, 
its essential purpose the multiplication of cells; the further his- 
tory of the egg is a description of the way in which the cells, con- 
stantly on the increase, arrange themselves in definite order, until 
they have gradually created, or, more truly, become, the adult 
animal. The object of embryology is to discover the laws ac- 
cording to which this arrangement is developed. 
We, of necessity, begin with a study of the process of segmenta- 
tion; but the details are so numerous that we can indicate only a 
few of them. The first result is the formation of two sets of cells. 
In one set the cells are small; in the other set they are large. 
Except in the sponges, the small cells form the outside covering 
of the body, appearing as a sac, or vesicle. The large cells form 
the lining of the digestive canal, or primitive stomach, and are, 
therefore, enclosed in the outer vesicle made by the small cells. 
It appears that this disposition arises in two entirely distinct ways. 
First, the cells formed by segmentation arrange themselves in the 
shape of a sphere, hollow inside, and its walls consisting of a con- 
tinuous layer of cells. One half is composed of small cells; the 
other half of large cells. Second, the result of segmentation is 
likewise a hollow sphere, but with double walls; the outer wall 
of small cells, the inner wall of large cells. In both cases the 
sphere transforms itself into a so-called gastrula. In the first in- 
stance, the large cells become inverted inwards, or, in technical 
language, invaginated; while the small cells grow down and 
around the others, until they encase them, leaving only a small 
opening, the primitive mouth. In the second instance, an open- 
ing breaks through both walls, thus making a mouth. This 
method of development is 
much rarer than the other, 
and unfortunately has never 
been studied in a thoroughly 
satisfactory manner. 
The accompanying figure 
A is a very simple form, such 
displays diagrammatically the — 7 | 
principal forms of gastrula. 
Fic ee coe of epia modi- aS occurs among Echino- 4 o 
fications W the Gastrula. cf. Text. A-E derms The adiiferenct = 
represent sections. 
cells is slight, but evident. In B, the difference is more marked, 
