55 
see a small cavity if one were present, and I do not think 
there is any reason to believe that the primitive digestive cavity 
becomes obliterated, although I am certain that this is the 
fate of its external opening. Before the crescent- shaped 
transverse groove has entirely disappeared, a small, irregular,, 
transparent body, Figure 36 s, makes its appearance at each 
end of it, and the subsequent history shows that these two 
bodies are the two valves of the shell, which are entirely sep- 
arate from each other from the first. 
THE KATE OF SEGMENTATION. 
Before I go on with the description of the later stages of 
development, I wish to discuss two or three points in connec- 
tion with the stages which I have already described ; one of 
these is the rate of segmentation. 
As I have already stated, the time record which I have 
given in connection with the figures is exceptionally slow, 
and I will now give the intervals between certain stages in 
the development of other lots of eggs for comparison : 
Lot A. — Warm, bright day. Eggs fertilized at 10 A. M. ; 
segmentation commenced between 12 and 1.30 P. M., averag- 
ing about 1 P. M., or three hours after fertilization. The 
stage shown in Figure 26 was reached by most of the eggs 
between 2 P. M. and 3 P. M., or about five hours after im- 
pregnation. 
The stage shown in Figure 32 was reached about 4 P. M., 
and seven hours after fertilization nearly all the embryos w T ere 
swimming at the surface. 
Lot B. — Cool day. Eggs fertilized at 10.30 A. M. About 
half of the eggs developed, and segmentation commenced he" 
tween 12.30 and 2 P. M., or about three hours after fertiliza- 
tion. The stage shown in Figure 26 was reached in about 
twelve hours, and the stage shown in Figure 32 w T as reached 
by a very few eggs during the second day, but at the end of 
the second day all were dead. 
