ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 383 
p. 577), that in a more advanced stage, probably after fecundation, 
the ovisac (which I called ovum) appears like a cellular mass. 
H. Miiller (Siebold and Kolliker’s Zeitschrift, iv. p. 331) speaks of 
the occurrence of yelk-division at this stage, without, however, 
describing that process more particularly. Vogt did not observe 
it, nor does Leuckart add much to our information on this head :— 
“T can say little more about it [yelk-division] than that it begins 
during the change of place of the ovum (as H. Miiller has also 
observed), and, as in the allied Ascidians, is a total yelk-division. 
When it has arrived in the feetal chamber (Brutsack), the yelk 
exhibits the well-known mulberry appearance” (4 ¢ p. 52). 
It is unfortunate that these observations are not so precise and 
detailed as they might have been ; for the question at once suggests 
itself, is this appearance presented by the ovisac really due to yelk- 
division? What has become of the epithelium of the ovisac? Might 
not the change in the appearance of the latter be due to an alteration 
in the character of the epithelium, similar to that which obtains in 
Pyrosoma ? 
The next steps in the development of Sa/pa are, as I pointed out 
in 1851 (2c pp. 575-577), the enlargement of the ovisac, the shorten- 
ing of its duct, and the consequent approximation of the ovisac to 
the atrial wall, and, finally, the protrusion of this part of the atrial 
wall into the atrium, so as to form a chamber containing the ovisac. 
This the German observers term the “ Brut-sack,” which may be ren- 
dered “ foetal chamber.” 
Arrived in the foetal chamber, I have said (4 ¢ p. 575) that the 
foetus “ becomes divided into two portions,—a larger turned towards 
the respiratory cavity, and which projects more and more into it, and 
a smaller, subspherical, turned towards and lying in the cavity of the 
sinus, and bathed by the parental blood.” The former becomes the 
embryo, the latter the placenta. 
Leuckart’s description comes to the same result, but is much 
fuller in details (pp. 52, 53) :— 
“ When the vitelline mass has increased to about double its primi- 
tive diameter, and has become changed by continual division into 
numerous small division-masses about ;3,” in diameter, it loses its 
spherical form. A circular constriction appears in it, by which its 
anterior end is marked off as a hump-like process. This constriction 
indicates the boundary between the foetus and the placenta. The 
placenta is, at first, the more considerable of these two parts. It is, 
one may say, the remains of the vitelline mass (yelk-sac) which is left 
after the formation of the rudiment of the embryo, and now, instead 
