384 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
of being directly applied to developmental purposes, is metamor- 
phosed 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 of a spheroid, it naturally possessed 
a convex surface; but this disappears as soon as the first traces 
of embryonic development are visible. The posterior end of the 
placenta becomes flattened, and its centre acquires a depression, 
which penetrates deeper and deeper into its substance. The placenta 
loses its originally solid character, and (even before there is any 
marked change in the embryo) becomes rapidly metamorphosed into 
a cupola-like structure, whose internal cavity is connected by its 
posterior aperture with the circulatory apparatus of the parent, and 
may be regarded as a sinus for its blood. The inner walls, freely 
bathed by this blood, exhibit many irregular elevations, which for 
the most part run, like ribs, from the apex of the cupola to its 
entrance. Not uncommonly there is also a conical process, which 
projects from the roof of the cupola for a greater or less distance 
into the cavity.” 
Vogt (Bilder aus dem Thierleben, p. 79 e¢ seg.) gives an essentially 
similar account of the development of the placenta of Salpa pinnata. 
Eventually the foetus makes its way through the wall of the atrium, 
and, carrying its placenta with it, lies free in that cavity, whence it 
must shortly be expelled. 
On the face of the matter, there appears to be a close analogy 
between this process and the development of the foetus of Pyrosoma ; 
for the projection of the atrial wall, caused by ovisacs in which the 
blastoderm is just appearing, may be fairly compared with the com- 
mencing fcetal chamber ; while, if there were only one ascidiozooid 
instead of four, its relation to the cyathozooid would be very similar 
to that which the embryo of Sa/pa has to its placenta. Nor is there 
wanting a very considerable resemblance in form and character 
between the cyathozooid and the placenta. 
But so much remains to be done before the developmental history 
of Sa/pa can be said to be fully made out, that I do not know how 
far these apparent resemblances may be depended upon as affording 
evidence of real similarity between the developmental histories of 
Pyrosoma and of Salpa. Vogt, Miiller, and Leuckart seem, as little 
1 On the other hand, the description and figures by H. Miiller, in the ‘Icones 
Zootomice’ of Prof. J. V. Carus, tab. 18, lead me to suspect the existence of differences in 
the development of the placenta in this species. 
