MUSEUM OF COMPARATIVE ZOOLOGY. llo 



untere kcirnig iind roth die obere dagegen glassartig iind homogen er- 

 scheint." A stage similar to his is figured in PI. II. Fig. 3. The mode 

 of formation of the cavity is not made evident in either ApostoHdes' 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 cav- 

 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 gastrula^ 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 amojboid 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. ApostoHdes, 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. ApostoHdes * 

 supposes that in Amphiura the hypoblast is formed by a delamination of 

 the blastodermic cells and not by invagination. Metschnikoff f 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 oesophagus. This, however, does 



* Op. cit., pp. 25, 192. 



t Op. cit., p. 14, PI. 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 



