216 PROFESSOR HUXLEY ON THE ANATOMY 
the inner tunic exhibits an elongated, discoid thickening, in which numerous small nu- 
cleated cells, like those in the ovisac and its duct, are to be detected. This description, 
it is obvious, would apply equally well to the young ovisac of Pyrosoma. 
It does not appear that the entrance of the spermatozoa into the duct of the ovisac has 
been observed in the Salpce, 
I have stated in my Memoir already cited (Phil. Trans. 1851, p. 577), that in a more 
advanced stage, probably after fecundation, the ovisac (which I called ovum) appears like 
a cellular mass. H. Muller (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 : 
" I can say little more about it [yelk- division] than that it begins during the change 
of place of the ovum (as H. Muller has also observed), and, as in the allied Ascidians, is a 
total yelk-division. When it has arrived in the foetal chamber (Brutsack), the yelk exhi- 
bits the well-known mulberry appearance " (I. c. 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 JPi/rosoma? 
The next steps in the development of Salpa are, as I pointed out in 1851 [l. c. 
pp. 575-577), the enlargement of the ovisac, the shortening 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 rendered " foetal chamber. 
Arrived in the foetal chamber, I have said (l. c. 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 primitive diameter, and 
has become changed by continual division into numerous small division-masses about 
! Jo'" 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 con- 
siderable 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 of 
being directly applied to developmental purposes, is metamorphosed into an accessory foetal 
organ. ... It has been mentioned above that the posterior segment of the yelk, in the 
foetal chamber, is freely bathed by the blood of the parent. By the delimitation of the 
embryo, this segment has now become the posterior end of the placenta : at first, as a part 
