524 
J. S. KINGSLEY. 
described, in Arthropod embryology, and tbe depression is the 
so-called germinal or ventral groove. In reality, at least in 
tbe case of Limulus, the lighter area marks the extension of 
the mesoblast (the epiblast extending entirely around the egg), 
and, in my opinion, the ventral groove is a modified blastopore. 
To this I will recur again. 
Larval Envelopes. — In this stage sections show that the 
epiblast cells have begun the secretion of the first larval enve- 
lopes, which Dr. Packard has endeavoured to compare with 
the amnion of insects. This comparison is utterly without 
foundation, as this is a cuticular not a cellular structure, the 
polygonal cell-like markings with which the surface is orna- 
mented being due to its mode of origin. Anticipating our 
account a little we will trace the history of what I would call 
the first larval cuticle throughout its history. In the stage 4, 
outside the mesoblastic area, the blastoderm consists of a 
single layer of large polygonal epiblast cells (fig. 39) resting 
directly upon the yolk. Each cell is covered with a thin 
cuticle which refuses to take any stain. This cuticle follows 
closely the contour of the surface of the epiblast, extending 
down between the various cells, thus giving rise to the poly- 
gonal markings which, in the later stages, are confined to the 
external surface, and which were so puzzling to Dohrn and 
Packard. 
At the time of the appearance of. the limbs this cuticle is 
nearly as thick as the epiblast, having increased by additions 
to the inner surface. The outer still retains its markings, 
but the inner is nearly smooth. As yet it is in close contact 
with the epiblast, the cells of which are still contributing to 
its increase (fig. 45). When the embryo arrives at stage 11 
the cuticle separates from the epiblast, and the cells of its 
surface have an average diameter of mm. ’0033. Of the 
explanation of the ensuing phenomena I am not certain, and 
the following is but tentative. It would seem that an osmotic 
action begins whereby water is taken in through the chorion 
and cuticle, thus creating a pressure which ruptures the 
