the Embryo of Purjmra lapiUus. 
27 
which they have respectively ingested. In some capsules I 
have found all the embryos to present nearly the same bulk ; 
whilst in others, I have met-with nearly full-sized embryos 
attached to the same conglomerate vitellus, together with 
embryos which had made but little advance upon their original 
dimensions. 
Another fact of no small importance, in reference to my 
view of the case, is that on crushing one of the pedunculated 
embryos, I have almost invariably found the small original 
vitelline mass in the interior of the large supplemental yolk ; 
the original vitellus being readily distinguished by its lighter 
hue, and by a very definite boundary line, although I could 
not find any evidence of a membranous separation. And 
again, in several embryos which had only ingested a very 
small allowance of the supplemental vitellus, this was seen 
not to envelop the original mass, but to remain superposed 
upon it (Plate IV., fig. 17). 
During the time in which they are thus appropriating the 
additional supply of nutriment which is required for their 
complete evolution, the embryos make very little advance in 
the development of new parts. This process soon recommences, 
however, and is then carried-on vigorously at the expense of 
the new material introduced. The ciliated lobes are much 
enlarged, and acquire those peculiarly long cilia, which have 
been distinguished as cirrhi ; the foot is evolved, and covered 
with short cilia; the auditory vesicles soon show themselves 
at its base ; the tentacula and eyes make their appearance ; 
and in the mean time the mantle and shell are being formed 
at the posterior part of the body. Now this developmental 
process may be arrested in any stage, for want of sufficient 
nutriment ; and thus we may not only meet with advanced 
embryos much smaller than the average size, in the same 
capsule with others of the usual dimensions, but may often 
find in the same capsule not only one embryo, but several 
embryos, in various stages of abortive evolution. The 
original vitellus, when it receives no addition from the con- 
glomerate mass, seems to be entirely exhausted by the de- 
velopment of the ciliated lohes, which attain nearly their full 
size, notwithstanding that the body behind them is no larger 
than at first, or may even have shrunk in consequence of the 
withdrawal of its contents ; so that such a ' monster ' would 
not be recognized as having any relationship to the ordinary 
embryo, but for the intermediate forms which are frequently 
to be met-with. I have even seen the auditory vesicles in one 
of these abortive embryos, which showed no sign of having 
received any additional vitelline material. But for the de- 
