tlie Embryo of Purpura lapilhis. 
29 
content myself for the present with having obtained what I 
hope will be deemed a satisfactory elucidation of it. I may 
mention, however, as points which require far more careful 
examination, the mode in which the first segmentation takes 
place in the true ova, and, more especially, the mode in which 
the primary mouth and oesophagus are formed. I am not at 
all sure whether this is effected by the tliimiing-aicay of the 
peripheral membrane, so as to form an entrance to the inte- 
rior ; or whether, as some appearances I have seen in later 
embryos would suggest (Plate IV., fig 13, «, Z>), the cavity 
which receives the additional vitellus is formed by the infold- 
ing of the peripheral membrane. The relation of the original 
mouth and oesophagus to the permanent organs of like kind, 
constitutes another very important subject of investigation, 
which becomes very difficult through the obscuration of these 
parts at a later period by the ciliated lobes, foot, &c. In re- 
gard to the history of later stages, also, I am satisfied that 
although Messrs. Koren and Danielssen have made out many 
important particulars, they have at the same time committed 
mistakes scarcely less serious than those which I have already 
had occasion to correct. For example, the curious contractile 
vesicle which they have described as the heart, is certainly 
not the real heart ; as this is not formed until some time after 
the contractile vesicle, and is situated deeper within the 
cavity of the mantle ; and the two may, under favourable 
circumstances, be seen pulsating simultaneously but not syn- 
chronously. 
It now only remains to inquire, whether any phenomenon, 
at all parallel to that which I have described, presents itself 
elsewhere in the animal kingdom. Something like it appears 
to take place, according to the statement of Siebold, in the 
case of the embryos of Planariae ; for he says of them,* that 
after they have a covering of ciliated epithelium, " they do 
not increase as before by the external fusion of cells ; but 
there is a muscular, discoid, oesophagus formed upon their 
periphery, through which the remaining cells are ingested 
and assimilated within the animal." It is very probable that 
many similar cases will be discovered, when the process shall 
have been adequately looked-for. In particular it may be 
expected to present itself among the Gasteropod Mollusca, 
since the enlargement of certain embryos at the expense ojf 
the other contents of their capsules appears to be a common 
phenomenon among them.t 
* Vergleichende Anatomie, § 129. 
f It seems reasonable to suppose, from tlie aoconnt given h}^ Messrs. 
