No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 31 
greatest in the second pair, and diminishing in depth from that 
point backwards. 
The embryos just described are in other respects nearly 
normal, the principal deviations being in the abbreviation and 
atrophy of the abdominal region in Figs. 8 and 10, and the 
modification of the cephalic lobes in Figs. 10 and 11. But 
invaginated appendages are frequently found in other types of 
embryos, as in Figs. 14, 16, 18, 33, 34, 40, 61, 65, 70, 98. 
Any appendage, except perhaps those of the first pair, may 
be invaginated ; the third pair appear to be most frequently 
affected in this way. 
Sections have been cut through several invaginated appen- 
dages, and confirm completely the conclusions drawn from 
surface views. They throw little further light on the sub- 
ject. Pl. X, Fig. 10, represents a longitudinal, vertical section 
through the embryo shown in Fig. 10. The only point of 
further interest here is seen at the inner end of the appendage, 
where the ectoderm seems to become continuous with the 
mesoderm, as though there was an inward cell proliferation at 
that point. If such were the case, a communication, through 
the hollow appendage, might be established between the exterior 
on one side, and the mesenteron on the other. A condition 
would then result like that which obtains in the gill slits of 
_ vertebrates. 
As to the cause of the invagination, very little can be said 
further than that it is a local, internal, rather than an external 
one. 
We cannot see how any of the general external conditions, 
such as the density or the composition of the surrounding 
medium, could produce such a purely local effect as the invagi- 
nation of one out of several apparently identical appendages. 
There is no evidence whatever that local pressure, such as 
that which the egg membranes, or the adjacent appendages, 
might exert, was the cause of the invaginations, for the mem- 
branes stand far away from the embryos at this period, and 
besides, an examination of most any of the figures shows 
clearly that the position of the invaginated appendages is such 
that the membranes could not touch them. Such cases as 
