NOTES ON THE EMBRYOLOGY OF LIMULUS. 
529 
rently undergo exuviation while thus concealed. The carapax 
splits along the frontal margin as in the adult. The first 
specimen moulted in exactly five weeks after hatching, while 
the others struggled along at intervals of one or two days, the 
last one of the lot casting its skin in seventy-two days after 
leaving the “ vicarious chorion.” After the moult the animal is 
much larger, and the most important change is in the increased 
length of the caudal spine, the general appearance being shown 
in Packard’s (’ 72 ) fig. 27 of plate v. For our purpose it is not 
necessary to carry the account of the external changes any 
further. Nowhere in the external development are there any 
startling metamorphoses, and nowhere, except in the closure of 
the blastopore and the post-oral position of the first pair of 
appendages, do we find facts which aid us in a discussion of 
the affinities or phylogeny of the Xiphosura ; certainly nothing 
which would indicate Crustacean relationships. 
Internal Development. 
The earliest stage of which I possess sections is that of fig. 4, 
but from difficulties of manipulation these are not very satis- 
factory, though they show some important points. The egg was 
cut transversely to the rudimentary blastopore, and one section 
is shown in fig. 44. The epiblast, a single cell in thickness, enve- 
lops the whole egg. The groove, which I regard as blastopore, 
is as deep as the thickness of the epiblast. Beneath this groove 
and extending to a short distance either side is the mesoblast, 
the limits of which correspond to the lighter area in fig. 4. 
This mesoblast arises wholly from the bottom and edges of the 
groove, and in the sections a rapid cell-proliferation is visible 
here. Beneath these two layers (epiblast and mesoblast) is the 
yolk. I believe that not only this groove but the primitive 
streak of all tracheates, as suggested by Balfour, is the homo- 
logue of the blastopore and that the yolk is wholly hypoblast. 
Schimkewitch (’ 84 ) comes to the same conclusions from studies 
of the spiders. How this modification arose I am not now 
ready to discuss, but when we consider the recent researches 
of Balfour on Peripatus, Hatschek on Amphioxus, and 
