523 
form body is composed, may at this time be plainly seen to be devel- 
oped in the interior of delicate spherical cells. The eight meridional 
bands have also by this time made their appearance, and may be 
seen in four pairs extending from within a short distance of the 
ocellus, along the sides of the embryo to about one-fourth of the 
whole distance from the ocellus to the mouth. 
The next stage observed is the differentiation of a portion of the 
large-celled tissue which constitutes the principal substance of the 
embryo, into two somewhat piriform masses, extending one on either 
side of the alimentary cavity, and which, simultaneously with their 
formation, become excavated by an extension of the large central 
lacuna. They thus form two large sacs appended to the central 
lacuna, and opening into it, and are to become the deep longitudinal 
gastro-vascular canals. 
Nearly at the same time, two other exactly similar masses, and 
also containing an extension of the central lacuna, are found oppo- 
site to one another, on the two remaining sides of the alimentary 
cavity, so that this cavity is now surrounded by four sacs. 
The two last-formed sacs now become divided, each into two, and 
the divisions thus produced become more and more widely sepa- 
rated from one another, and become superficial to the first formed 
sacs. We now find them in contact with the peripheral layer of the 
embryo, where each sac corresponds to the space occupied by a pair 
of meridional bands and the interval between them. 
Lastly, These four sacs begin to divide, each into two, from the 
peripheral towards the central side, leaving one half adhering by its 
peripheral side to each meridional band, so that every band has now 
a sac to itself, and the subsequent conversion of these sacs into the 
eight meridional canals is easily understood. 
About the same time the central lacuna has sent off its two 
branches, which are to open by external orifices at each side of the 
ocelliform body ; but the author had not succeeded in satisfactorily 
tracing the formation of these branches. 
Hitherto the embryo was confined within the shell, but in the 
next stage described, the young Beroe was free. It was about one- 
eighth of an inch in length ; the meridional bands had extended to 
somewhat more than half the distance between the ocellus and the 
mouth. The mouth was surrounded by the circular canal, and the 
two deep gastro-vascular canals had opened into the latter. The 
