70 
In from two to four days after fertilization, the embryo as- 
sumes the form shown in Figure 38, which is a view of the 
right side. The shell is now large and regular in outline, and 
covers nearly half of the surface of the body. The digestive 
tract now has two external openings, m and an, which are 
close together on the ventral surface of the body. In a side 
view, Figures 38 and 39, it was almost impossible to say 
whether either or both of these communicate with the diges- 
tive cavity, hut embryos were frequently found with the two 
valves of the shell stretched out into the same plane, and with 
the body pulled up and flattened against the shell, and in a 
dorsal or ventral view of such an embryo, it w T as easy to see 
that both openings do communicate with the central cavity. 
Figure 41 is a dorsal view of an embryo at the same stage 
as Figure 38, but with its valves extended. 
The stomach is seen through the shell, and when the ani- 
mal is in this position, is pear-shaped, with the broad end 
of the pear in front and the narrow portion on the median 
line behind, and the anus, a, could be seen at the tip of the 
stalk, ^while the mouth, mo, is in the centre of the broad an- 
terior end. 
It will be seen that these openings are much farther apart 
when the body is stretched by the opening of the valves than 
when these are more inclined to each other, as they are in the 
side view, Figure 38. The anterior opening, or mouth, 38, 39 
and 42, mo, is not, at this stage, a simple tube, but a large, 
nearly spherical pouch, which is capable of protrusion from 
the surface of the body. I was not able to determine whether 
these two openings were or were not formed by the division 
of the single opening shown at the stage 37, and therefore 
cannot say whether the opening in this figure is the mouth, 
the anus, or both. 
In an embryo four or five days old, Figures 42 and 44, the 
digestive tract has increased considerably in size, and the 
posterior portion is much smaller than the anterior part, and 
forms a tubular intestine, i. At the stage shown in Figure 
44 the lateral walls of the stomach begin to fold out, so as to 
