MUSEUM OF COMPARATIVE ZOOLOGY. 117 



readily distinguished from each other. The ventral side is quite flat, the 

 dorsal more couvex. 



Looking through th'i 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, PI. I. 

 fig. 1 6, is now in the gastrula stage. 



At the pole of ^invagination in the region of the coeloraa, 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, PI. 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 larvae of Ophiopholis.* 



On the second day, PI. I. fig. 16, after the fecundation of the Ophio- 

 pholis, it was observed that the invaginated end of the stomach becomes 

 somewhat inflated, PI. I. fig. \Q, 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 

 Echinoderra 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 Opliiurans, notwith- 

 standing from a 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 wliich occur at iliLs point very difficult. Nachtrieb seems 

 to have had a similar difficulty in the genus, Ophiophragma. 



t Op. cii., p. 199. 



