MUSEUM OF COMPARATIVE ZOOLOGY. PLT 
readily distinguished from each other. The ventral side is quite flat, the 
dorsal more convex. 
Looking through the larva with its infolded outer layer of cells or 
hypoblast, from the ventral side, we notice that the infolding has pro- 
ceeded about two thirds the axial length of the larva, and formed a fun- 
nel-like tube. This tube is the hypoblast, the primitive stomach, and 
at the pole of infolding is situated a mouth, or. The whole larva, Pl. I. 
fig. 16, is now in the gastrula stage. 
At the pole of invagination in the region of the cceloma, between the 
infolded walls and the external crust of cells, epiblast and hypoblast, two 
masses of cells, a cl, are situated, one on each side, which are the 
mesoblastic cells already spoken of. These cells are spherical, stellate, 
branched, or elongate. The walls of the anterior pole of the gastrula are 
more densely pigmented than the remaining parts of the larva. The 
pigmentation is most dense on each side of the mouth. When the same 
gastrula is seen from one side, Pl. I. fig. 15, it is noticed that the infolded 
archenteron does not hang exactly in the longer axis of the larva, but that 
the closed end approaches the ventral side. Its extremity has a tendency 
from the very first to draw near the ventral wall. It approaches so near 
that it may be supposed to be met by a second infolding, through which 
an opening may be formed. I have not observed this second invagina- 
tion, or this opening to be formed ; although the general law of Echino- 
derm development would call for such an occurrence. I did not observe 
a second opening to be formed in the larvee of Ophiopholis.* 
On the second day, Pl. I. fig. 16, after the fecundation of the Ophio- 
pholis, it was observed that the invaginated end of the stomach becomes 
somewhat inflated, Pl. I. fig. 16, g a, by an enlargement of the cavity. 
Although this inflation has not been traced farther, and water tubes 
were not seen to arise from it, as we know takes place in the course of 
Echinoderm development, up to this point the modifications in this 
region of the archenteron closely resemble similar formations observed 
by others in the echinoid pluteus. The origin of the water tubes from 
the primary invagination is yet to be observed in Ophiurans, notwith- 
standing from @ priori grounds we suppose such to be the case. All 
embryologists, however, do not accept such an explanation. According 
to Apostolides,t who has written the last important work on the devel- 
* The clustering of cells in the cavity of the larva made accurate observations 
in regard to the changes which occur at this point very difficult. Nachtrieb seems 
to have had a similar difficulty in the genus, Ophiophragma. 
+ Op. cit., p. 199. 
