20 MR. E. RAY LANKESTER ON THE 
Plate 6. fig. 31. The operculum appears on the lower surface of the whole length 
of the foot (op), and the mouth (0) is commencing to break through the pharyngeal 
plug. ‘The otocyst (ot) is now seen to have become quite detached from the epiblast in 
which it originated. The original vacuolar cavity is surrounded by regularly disposed 
columnar cell-elements. It is still free from any solid contents. 
Plate 6. fig. 32 is a portion of the body-wall more highly magnified, to show (fw) 
fusiform cells lying just below its surface, which appear to be the muscular elements of 
the parietes. 
Plate 6. fig. 33 represents the left face of such an embryo as that of fig. 31. It 
is focused high, so that the surface of the alimentary mass is in view, and also the 
surface of the cephalopedal region. This brings into view the fold dv (descending 
border of the velum), by which the edge of the velum is continued on each side on to 
the foot. ‘The embryo is now becoming markedly unilateral in its external features, as 
may be seen by comparing the next figure with the present. 
_ Plate 6. fig. 34 represents a similar view to that given in fig. 33, but now it is of the 
right side of the embryo. It is on this (the right side), as in other Nudibranchs, that 
the two cells already so early distinguished (mn) develop into a prominent mass, at 
which point subsequently the anal termination of the alimentary canal develops, and 
from which there grows also a fold which partially overlaps the shell in this region, and 
increases in extent so as to form the rudiment of a mantle-flap. 
Plate 6. fig. 55 is a detached cephalopedal mass, or velum and foot, of the phase 
represented in fig. 29. The horseshoe-shape of the velum with the mouth (0) lying 
in its hollow is well exhibited. 
Plate 6. fig. 56 represents an embryo further advanced. ‘The shell is not here seen 
of its proper proportionate size, on account of the position into which it has slipped. 
The marked advance in this embryo consists in the clear definition of the cavity of 
the double gastric sac (a/), and its attachment to the body-wall by transverse muscular 
fibre-cells. 
Plate 6. fig. 37 is not quite so far advanced ; but greater detail is given of the cephalic 
regions. In particular, fibres are seen passing backwards from the neighbourhood of 
the cephalic ganglion (nq), which is now large. They may be muscular attachments to 
the pharynx or nerves. 
In Plate 6. figs. 36, 37, the furthest stage to which I have traced A. major is 
given. The history so far furnishes interesting data for comparison with A. minor 
and with other Mollusca, though I have fully stated the doubts and guesses connected 
with the interpretation of much which is figured. In this, as in other cases, the figures 
must at any rate serve as a basis of fact, interpret them how we may. In a subject so 
vague and tentative as the embryology of the lower animal classes must for some time 
remain, the best contribution which one can expect as yet to offer towards unravelling 
the complicated phenomena, is the observation and record of fact—a contribution which 
can best be effected by few words and copious drawings. 
