the Embryo of Purpura lapillus. 
23 
Danielssen in the ovum of Buccinum ; and in a supplement 
to their former researches, they confirm the statement of pre- 
vious observers, that in the youngest ova they exist in the in- 
terior of the egg, and pass from thence to the surface. They 
distinctly state, however, that they could not find them in the 
ova of Purpura, although they took great pains to look for them. 
There frequently appears to be but one (Plate III., fig. 3) ; but 
this is probably because the second vesicle is behind the first. 
When both come into view, one vesicle is almost invariably 
observed to be smaller than the other (Plate III., fig. 4). It is 
comparatively rare for three to present themselves. The 
vesicles are very pellucid; the larger one is filled with a fine 
granular material, and apparently unfurnished with a nucleus ; 
the smaller is a nucleated cell, and is more transparent. The 
vesicles are invariably found on that side of the ovum, at 
which the head and other appendages are formed ; never at its 
posterior part. Their gradual separation from the yolk-mass, 
to which they seem to be connected by a kind of viscid mucus, 
clearly shows that this is not enclosed by any vitelline mem- 
brane.* 
The difference between the two kinds of bodies, at first in- 
distinguishable from each other, now becomes apparent ; the 
former are nothing else than vitelline spheres (as was suspected 
by M. Milne Edwards in the case of Buccinum), | whilst the 
latter are time ova. The subsequent destination of the two 
respectively makes this very obvious. 
Shortly after the completion of the subdivision of the 
' vitelline spheres' into clusters of ' yolk-segments,' the whole 
aggregate of these shows a tendency to mutual coalescence. 
Up to that time, these clusters have remained distinct from 
each other, and are always separated by the intervening viscid 
liquid ; and even if flattened against one another by pressure, 
they separate again on the removal of that pressure, without 
the least appearance of mutual fusion. But at this period they 
adhere with considerable tenacity, so that it is difficult to tear 
apart the aggregate vitellus into its component clusters of 
yolk-segments (Plate IV., fig. 7) ; and this coalescence speedily 
becomes so complete, through the mutual adhesion of the 
yolk-segments themselves, that the form and demarcation of 
the clusters are obscured and finally lost ; the resultant being 
a conglomerate mass of yolk-segments (Plate III., fig. 10) ; 
* For the knowledge of the facts stated in this paragraph, I am indebted 
to Mr. G. Busk. I had myself frequently noticed the presence of the 
vesicles in the course of my examinations of the contents of the capsule 
at this stage ; but I was not then aware of their significance. 
t ' Ann. des Sci. Nat.,' tom. xviii., p. 261, mjU. 
