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 deyelopment are visible. The posterior end of the placenta 

 becomes flattened, and its centre acquires a depression, Avhich 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- 

 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 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 Fijrosoma; 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 Salpa 

 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 afPording evidence of real similarity between the developmental 

 histories of Pyrosoma and of Salpa. Vogt, MiiUer, 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 Tyrosoma 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 details 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. Miiller, in the ' Icones Zootomicae ' of Prof. J. V. Cams, 

 tab. 18, lead me to suspect the existence of differences in the development of the placenta in this species. 



