MUSEUM OF COMPARATIVE ZOOLOGY. its 
untere kornig und roth die obere dagegen glassartig und homogen er- 
scheint.” A stage similar to his is figured in Pl. II. Fig. 3. The mode 
of formation of the cavity is not made evident in either Apostolides’ or 
Metschnikoff’s account. The inner layer of cells, or the red cells, is sup- 
posed to be the mesoblast ; the outer, the epiblast ; and the walls of the 
cavity, or hypoblast, are concealed by the mesoblast. It is possible that 
hypoblast and mesoblast have as yet not differentiated themselves in the 
red cells. The blastosphere unquestionably has a true segmentation cay- 
ity, but by the concentration of the pigmented cells near its interior, by 
which concentration the superficial layer (epiblast) of transparent cells 
appears to be separated, renders the interior of the egg so opaque that ob- 
servation of the contents is almost impossible without cutting sections. 
As only one blastosphere and not more than four gastrule were found, 
my material was limited. The transparent outer layer (ep) is probably 
the epiblast, and the masses of reddish cells in the interior of the body of 
the embryo may be the same as the so-called amoeboid or mesoblastic cells 
in other Echinoderm embryos. The true hypoblast either is not sepa- 
rated from the red cells or is differentiated from them and hidden by 
these opaque cells. There is what appears to be an external opening (a) 
passing into the opaque region of the ovum, which fact would seem to 
indicate the existence of a cavity. Unfortunately, however, I am not sure 
that such an opening exists. Apostolides, however, has described and 
figured this opening, and there is no reason to doubt its presence. It is 
necessary to have new observations on the existence of this primitive 
opening and the way.it is formed. It disappears early in larval life, 
The mode of formation of the hypoblastic wall of the cavity of the gastrula 
of Amphiura has been interpreted in two ways. The first, and that 
which would seem the true one from what we know of other Echino- 
derms, is the embolic method; the second by delamination, unknown 
elsewhere in the group, is very exceptional and peculiar. Apostolides * 
supposes that in Amphiura the hypoblast is formed- by a delamination of 
the blastodermic cells and not by invagination. Metschnikofft says that 
it is formed by invagination. It is probable that the true blastopore was 
not observed by this naturalist, and the invagination which he observed 
was that of the mouth and possibly the esophagus. This, however, does 
* Op. cit., pp. 25, 192. 
+ Op. cit., p. 14, Pl. III. Fig. 6. A cavity with hypoblastic walls between it and 
the cutis (c) must have existed, as he speaks in the text of such a cavity (*‘ Darman- 
lage”). His figures do not well represent the hypoblast. 
VOL. XIII. — No. 4. 8 
