374 DR. ALLMAN ON THE ANATOMY AND PHYSIOLOGY OF CORDYLOPHORA. 
menon of yelk-cleavage may be distinctly perceived in them, fig. 16. In the mean 
time a delicate structureless pellicle (fig. 13 e) has been formed on the external surface 
of the cellular sac ; it is evidently a mere secretion from the surface of the sac, 
resembling the secretion of the polypary from the ccenosarc. Soon after the commence- 
ment of segmentation of the vitellus, the cellular sac with its system of ramified tubes 
disappears, and the cluster of ova is now distinctly exposed (fig. 15) lying upon the 
diverticulum from the ccenosarc, and immediately enclosed by the delicate structure- 
less sac formed by the pellicle which had been secreted on the outside of the cellu ar 
sac, while the whole is included within the external pergamentaceous investment 
from the polypary. The ova vary in number ; in many capsules I have counted ten, 
twelve, or even more, while some few contained only two or three. 
The mulberry-like condition of the ova at length disappears, and these bodies now 
begin to elongate themselves, and when viewed by transmitted light, present a trans- 
parent margin (fig. 17). As development advances the ova become more and more 
elongated, and they soon exhibit a slow but evident motion m the interior of the 
containing sac (fig. 1 8). They are now in the condition of free embryos, only waiting 
for the rupture of the enclosing structures to escape into the surrounding medium 
and enjoy an independent existence. We accordingly soon find that these structures 
no longer confine them, and the embryos may now be seen escaping from the torn 
summit of the capsule in the form of infusorial animalcules clothed with short cilia, 
and swimming freely through the surrounding water (fig. 19). 
The embryos on their escape from the capsule are usually of an elongated oval 
figure (figs. 19, 20), but very contractile, and capable of undergoing considerable 
change of form ; they frequently assume a pyriform shape (fig. 21), but they never 
appear compressed, and the comparison with a Planaria, to which Dalyell and 
others have likened the embryos of Campanularia, will not here strictly apply. 1 my 
are included in a distinct ectoderm, and contain in their interior a cavity which is 
separated from the ectoderm by a cellular interval representing the endoderra of the 
adult zoophyte, but in which the brown granules have not yet shown themselves. 
No mouth is yet evident, and the ectoderm seems in this early stage destitute ot 
t tl C 0 I. i S • 
After the embryo has continued for some time in the condition now described, the 
period of its final fixation approaches ; the cilia disappear ; one extremity becomes 
expanded into a kind of disc, by which it soon attaches itself to some fixe, o ject, 
the mouth has now become apparent, and thread-cells are developed in the ectoderm 
(fiff. 22) ; the embryo increases in length and thickness, and from the free extremity, 
which has begun to assume a clavate form, a single series of tentacula soon shoot 
forth (fig 23) ; these are about four in number, and are situated a little behind the 
anterior end; the portion of the body which lies before them becomes the post- 
buccal cavity of the adult; the gastric cavity has become prolonged through the 
young stem ; a delicate polypary has begun to invest it ; and its walls begin to be 
