676 OBSERVATIONS ON THE EMBRYOLOGY OF LIMULUS. 
that the blastodermic skin, just before being moulted, consisted of 
nucleated cells, and also traced its homology with the so-called 
amnion of insects. I have this summer, by making transverse 
sections of the egg, been able to study in a still more satisfactory 
manner these blastodermic cells and to observe their nuclei before 
they become effaced during and after the blastodermic moult. 
On June 17th (the egg having been laid May 27th) the periph- 
eral blastodermic cells began to harden, and the outer layer, that 
destined to form the + amnion,” to peel off from the primitive band 
beneath. The moult is accomplished by the flattened cells of the 
blastodermic skin hardening and peeling off from those beneath. 
uring this process the cells in this outer layer lose their nu- 
clei, and, as it were, dry up, contracting and hardening during 
the process. This blastodermic moult is comparable with that of 
Apus, as I have already observed, the cells of the blastodermic 
skin in that animal being nucleated. 
This blastodermic skin in its mode of development may also 
safely be compared with the “amnion” of the scorpion as de- 
‘scribed and figured by Metznikoff, and we now feel justified in un- 
hesitatingly homologizing it with the “ amnion ” of insects, in which 
at first the blastodermie cells are nucleated, and appear like those 
of Limulus. Moreover the layer of germinal matter, from which 
the blastodermic skin moults off, may be compared with the prim- 
itive band of insects. On June 19th, in other eggs, the cells 
of the blastodermic skin were observed to be empty, and the nu- 
clei had lost their fine granules, and were beginning to disappear. 
The walls of the cells had become ragged through contraction, 
and in vertical section short peripheral vertical radiating lines 
could be perceived. 
At this time an interesting phenomenon was observed. In cer- 
tain portions of the blastodermic skin, or amnion, the cells had 
become effaced, and transitions from the rudiments of cells to 
those fully formed could be seen. From this we should supposé 
that the retention of these cells in the amnion of Limulus is due 
to the singular function this skin is destined to perform, i.e., to act 
as a vicarious chorion, the chorion itself splitting apart and falling 
off in consequence of the increase in size of the embryo. In in- 
sects these cells disappear, and after the skin is moulted it appears 
structureless. 
From studies afterwards carried on in the laboratory of the 
