30 
On the Embryo of Purpura lopillus. 
But I think that we may reasonably question whether the 
process, anomalous as it seems, is really anything else than a 
variation of the mode of introducing into the body of the 
embryo that separate store of " food-yolk," which many of 
the higher forms of animals require as a necessary addition 
for the completion of their embryonic development, that has 
been at first carried on at the expense of the " germ-yolk." 
Thus in the early development of the chick, the cicatricula or 
germ, formed (as appears probable) by the cleavage of the 
germ-yolk, sends off a membranous expansion, which extends 
itself by marginal growth, until it spreads itself around the 
food-yolk, which it thus encloses in what may be termed the 
temporary stomach of the embryo. And it seems to me that 
there is an essential parallelism between the condition of the 
embryo-chick at this stage, and that of the embryo-purpura 
when it has distended its body with yolk-segments which it 
has detached from the conglomerate mass, and has conveyed 
into its own visceral cavity. 
The only difficulty that occurs to me, in regarding the great 
mass of the contents of the capsules as yolk- segments, arises 
from the fact of their undergoing a subdivision like that of 
fertilized ova. But this subdivision has very definite limits ; 
it never tends, like the subdivision of the yolk of the true 
embryos, to a " differentiation" of parts ; and seems to be a 
mere " fractionnement," designed to break up each body into 
smaller spherules. 
Koren and Danielssen of the development of Buccinum, that they have 
made a like error of interpretation to tliat which they have committed in 
the case of Purpura ; of this Professor Milne-Edwards has already ex- 
pressed his suspicion. See ' Ann. des Sci. Nat.,' 3ieme S^r., torn, xviii., 
p. 261. 
