24 
MR. E. RAT LANKESTER ON THE 
rudimentary condition. All the deformed embryos agreed in this — that the yellow yelk- 
spheres were gone ; whether they had been extruded (as I think most probable), or 
whether they had been absorbed, I could not determine. In that drawn in figs. B, C, 
and in others a simple yellow oily-looking body (ry) appeared to be the only remnant 
of the yellow spheres; and from its position it suggests that the rest had escaped 
through a rent in the epiblast. The mouth in these deformities was open, the alimen- 
tary cavity complete and lined with cilia, its walls nevertheless quite free from any 
of the yellow granules of the residual yelk. A peduncle of apparently solid, tissue (B) 
passed from the lower part of the gastric sac to the side of the large thickened “ shell- 
patch.” The condition of the shell-patch, as exhibited in the specimen figured and 
in others, was most important ; for it had produced a thickened and brown-coloured 
(chitinous 1) shell of small area, but relatively great solidity. A button or knob (pi) 
continuous with this thick disk-like shell occupied the groove or indentation of the 
shell-gland, forming thus an enclosed plug. Thus the real significance of the shell- 
groove of the embryo is demonstrated by a pathological condition artificially induced. 
In a subsequent part of this memoir will be found the description and figure of a 
similar chitinous plug in connexion with the earliest rudiment of the shell in Neritina 
Jluviatilis, which I studied at Oxford in May 1873. 
The development of the alimentary tract in these deformities, in the absence of the 
two yellow yelk-spheres, seems to show that it is independent of them in origin, its 
ciliated lining being derived elsewhere than from material furnished by them. And, 
again, the separate position and solid condition of the intestinal piece marked It agrees 
well with what has been put forward above as to the origin of the two parts of the 
alimentary canal. The gradual pushing of the ciliated lining of the gastric cavity (al) 
along the solid piece B would give an intestine lined by “ hypoblast ” and built up 
exteriorly by mesoblastic muscular elements. 
Plate 7. figs. 10 & 16 show two planes of one embryo, the pigment-spot (pg) being 
introduced into each as a fixed point of comparison. The foot and velum are now 
taking definite shape, the former already provided with a very delicate operculum. 
From the anterior horizontal border of the velum a fold ( dv ) descends on each side of 
the foot as in A. major. In the deeper view (fig. 10) the letters int mark a part of 
the cell-mass ( pme ) of fig. 11, now assuming development as part of the alimentary tract. 
The figures on Plate 8 chiefly illustrate what can he ascertained of the development 
of the alimentary canal. 
Plate 8. fig. 18 is the most rudimentary shell, discoid in form, with an irregular 
surface, hyaline and exceedingly delicate in texture. 
Plate 8. fig. 19 is the shell-patch as detached by pressure sufficient to break the 
embryo. The same structure was figured from A. major in Plate 5. fig. 21. 
Plate 8. fig. 20 represents an embryo (of the same lot as that drawn in fig. 17) seen 
from behind in such a position that the posterior border of the velum forms its upper- 
boundary. Seen through is the pharynx ( ph ), and on each side (also seen through) 
are the pigment-spots (pg). 
