the Embryo of Purpura lapillus. 
25 
to coalesce ; whilst, on the other hand, I have found the 
coalescence to have completely taken place, without any sign 
of more advanced embryonic development, than the presence 
of the clear margin around some of the vitelline bodies im- 
bedded in the aggregate mass. Generally speakino;, however, 
I believe that by the epoch of complete coalescence of the 
vitelline spheres, some of the embryos will have attained the 
oesophageal stage, and will be found external to the conglomerate 
mass, as shown in Plate IV., fig. 7, ^, c, e^fg; whilst others, 
«, not yet so far advanced, are imbedded in its superficial por- 
tion, and are consequently only to be found by careful search. 
That as many of these original embryos as usually come to 
maturity, are to be found in some stage of development or 
other, in every capsule whose vitelline segments have coalesced, 
except in such as are destined to failure from the want of 
them,* I am satisfied, from the unequivocal evidence afforded 
by repeated observation. 
The embryos which have attained the stage of development 
just described, attach themselves by the mouth to the con- 
glomerate yolk-mass (Plate III., fig. 10) ; and by the continued 
action of their ciliary apparatus the particles of this mass are 
detached, and are driven down their (Esophagus into the gas- 
tric cavity, which is gradually distended by them^ until the 
embryo attains many times its original size. This statement 
is not a mere inference, resting on the fact that embryos of 
various sizes are often found attached to the same yolk-mass 
(Plate IV., fig. 11, a, Z>, c, 6^f)i for under favourable cir- 
cumstances I have been able to witness the detachment and 
ingestion of the particles, and to observe a sensible enlarge- 
ment of the embryo in consequence. This, however, is not 
often possible, since the entrance into the oesophagus is gene- 
rally overlapped by the conglomerate yolk-mass on one side 
or the other, so that it cannot be seen-through. On one occa- 
sion, however, I witnessed a very curious occurrence, which 
afforded a remarkably satisfactory view of the process. 
Having opened a capsule, of which the contained embryos 
liad almost entirely appropriated the yolk-mass, and had con- 
sequently attained to nearly their full size, I observed one 
* The number of true ova bears no relation whatever (as will hereafter 
appear) to that of the vitelline spheres. Sometimes there are not enough 
of the latter for the full development of the embryos ; sometimes, on the 
other hand, there are more than are required. And I have found so many 
capsules in which the vitellus was undergoing decomposition, apparently 
for want of embryos to apj^ropriate it, interspersed among those which 
contained the usual number of embryos in full course of development, 
that I am satisfied that the want of true ova is not a very unfrequent 
occurrence. 
