82 WILLIAM BATESON. 
will I hope shortly appear, I propose to call B. Brooksii. My 
attempts at rearing this species from the egg were unsuccessful, 
and I was prevented by illness from making any additional 
observations on its development. 
On leaving Beaufort, N. C., I returned to Hampton, Vir- 
ginia, in the beginning of September, in the hope of finding 
the later stages in the development of B. Kowalevskii, and 
was fortunate enough to procure a complete series of larve 
from Stage H to individuals possessing ten gill-slits, in which 
condition the generative organs are first present. It is intended 
in this paper to give a general account of these stages, together 
with the histology of the animal, until two pairs of gill-slits are 
developed (fig. 1). From this point the further histological 
differentiation of the various organs will be described under 
separate headings. 
With this account of the organogeny of B. Kowalevskii 
will be given, as far as possible, a comparative description of 
the same parts in all the species which I have hitherto 
examined, 
External Changes.—The larva of B. Kowalevskii was 
described (loc. cit.) as leaving the egg in an elliptical form, 
divided by two transverse constrictions into three segments. 
The surface of the body was ciliated, a special tuft of long 
cilia being developed at the anterior end, while the posterior 
region was surrounded by a transverse band of long cilia. 
From further observations it seems probable that this period 
(Stage D) assigned as the time of hatching is too early ; for 
embryos kept in aquaria do not break the membranous shell 
before Stage G is reached. Probably, therefore, the larve 
found swimming in Stage D had escaped owing to an artificial 
rupture of the shell during the process by which they were 
found, an account of which is given in an appendix. 
The formation of the mouth as a ventral pore in the anterior 
groove was described (No. 3, fig. 41). It opens directly into 
the archenteron, which was previously a closed sac, from which 
five mesoblastic pouches had been given off, forming the 
anterior, middle, and posterior body cavities respectively, 
