AND DEVELOPMENT OF PYROSOMA. 247 
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 un- 
1 commonly 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 et seq.) gives an essentially similar account 
of the development of the placenta of Salpa pimiata *. 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 foetal 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 Scdpa 
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 Salpa 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 as my- 
self, to have endeavoured to trace the fate of the ovisac and of its epithelium. And yet, 
" with the development of Pyrosoma before me, it is impossible to arrive at a conclusion 
in the absence of information on this head. The long retention of the foetus of Salpa in 
connexion with the parent and nourished by its blood, in contrast to the early separa- 
tion of the foetus of Pyrosoma and the turning of its cyathozooid to account in another 
way, leads me to conceive that considerable differences will be found in the detads of 
their development, though I suspect further inquiry will prove that, in essentials, they 
are very similar. 
On the other hand, the description and figures by H. Muller, in the ' Icones Zootomies* ' of Prof. J. V. Cams, 
tab. 18, lead me to suspect the existence of differences in the development of the placenta in this species. 
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