Ova of the Nudibranchiate Mollusca. 387 
structures are much more opake. The loose membrane surround- 
ing the body at a later period (fig. 20 v) adheres to the inner sur- 
face of the shell, and like the other soft textures and the retractor 
muscle, contains numerous small cells or nuclei. At this period 
the embryo was never seen to draw the ciliated discs within the 
shell. 
The ova of the other Nudibranchiate Mollusca examined passed 
through the same stages of development as those of the Doris 
bilamellata, and the embryos presented, with some slight modi- 
fications in size and position, the structures we have described. 
The embryo of the Doris tuberculata, at the time it leaves the 
case-membrane, is larger than the others, measuring about 1-100th 
of an inch in length and 1-130 in the antero-posterior direction ; 
that of the Dendronotus arborescens measured 1-165 in length, 
and 1-250 in the antero-posterior direction ; and that of the Doto 
coronata about 1-200 in the former and 1-260 in the latter di- 
rection. The shell of the embryo of the Doris tuberculata is re- 
latively shorter in length or in the vertical direction than in the 
other embryos examined, and the parts which protrude beyond 
the shell are not only positively but relatively larger. The ciliated 
dises are especially large, the apex of the foot is narrower and 
more pointed, and the transparent cells in the base of the foot 
are relatively smaller. The gullet and stomach are short, and 
the two large cells at their lateral surfaces are placed near each 
other ata later period of its development than in the D. bilamel- 
lata. The cells at the termination of the intestine are more nu- 
merous and transparent, and occupy a considerable space of the 
upper part of the right side of the shell, so that the body of the 
embryo lies more to the left than to the right side of the mesial 
line of the shell. . 
The cilia were not observed on the upper part of the ovum of 
the Goniodoris Barvicensis and Polycera quadrilineata until the 
seventh day, or one day later than in that of the D. bilamellata, 
and the embryos of the Polycera did not begin to leave the spawn 
until the eighteenth day. The transparent cells in the base of 
the foot are, from the less opacity of the body, seen at an earlier 
stage in the embryos of the Polycera (fig. 14.2), and in a great 
number of these ales at this stage, an opake irregular patch, com- 
- posed at least partly of aggregated cells, lay on the surface of the 
lower end (fig. 146). The development of all the ova of the 
same spawn does not proceed pari passu, but much greater irre- 
gularities were observed in the ova of the Polycera quadrilineata 
than in the others; for in many of these the bipartite division 
did not even proceed regularly, nor were the cells into which it 
divided of the same size. That these irregularities were not 
entirely dependent upon the artificial conditions under which 
