AND DEVELOPMENT OF PYROSOMA. 245 



8, 9, 10. Thus far the problem has been to find a parallel for those early embryogenetic 

 processes which are ordinarily common to large assemblages of living beings. Analogies 

 for the more special modifications which the blastoderm undergoes may be sought for in 

 the group of which the genus Pi/t'osoma forms a part. In the first place, it may be asked, 

 are there, in this group, any examples of the division of the blastoderm into segments, one 

 of which is to serve a temporary purpose, while the others become ascidiozooids ? 



Leaving the development of the caudal aj)pendage of ordinary Ascidians out of con- 

 sideration, as hardly a case in point, it yet appears that even in these Ascidians, 

 the body of the embryo is, during its locomotive stage, divided into two segments, the 

 anterior of which gives rise to the so-called suckers (which are diverticula of its wall 

 with involuted ends), while the posterior is the rudiment of the body. 



Lowig and Kolliker, in their description of the compound larva of Botrylliis (originally 

 discovered and described by Sars), consider the three processes which are given off from 

 the "large round 'mamelon' provided with an orifice" as the homologues of the three 

 processes given off from the anterior division of the larval body in the simple Ascidians. 

 In this case this ' mamelon,' which they consider to be the rudiment of the cloaca, must 

 correspond with that anterior division. But the examination of their figures and descrip- 

 tions renders it hardly doubtful to my mind that the ' mamelon ' is a structure homo- 

 logous with the cyathozooid of the foetal Pyrosoma, the eight rudimentary ascidiozooids 

 of the Botrylliis being arranged around its base, just as the four are disposed in the foetal 

 Pyrosoma. If this reasoning be correct, it follows that the cyathozooid of Pyrosoma 

 corresponds with the anterior division of the body in the ordinary Ascidian larva, e. g. of 

 Clavelina. 



The peculiar connexion of the embryo Pyrosoma with its ovisac, and the extrusion of 

 the latter combined with the embryo, as a single foetus, into the mid-atrium of the 

 parent, are, however, peculiarities for which we should in vain seek a parallel among ordi- 

 nary Ascidians. But there is one family of this class, the Salpm, which resemble Pyrosoma 

 in having an elgeoblast, and in possessing no caudal appendage in the larval state (differing 

 in the same respects from all other Ascidians), in which the search for analogies is more 

 hopeful. 



Most SalpcB, like Pyrosoma, possess, as Krohn was the first to point out, but a 

 single ovisac, connected by a peduncle-like duct with the wall of the mid-atrium. Prof. 

 Leuckart, who, with a knowledge of all that had been written upon the question, subjected 

 the reproductive processes of the SalpcB to a renewed and very careful scrutiny, some 

 years ago * stated {I. c. p. 47) that the ovum of Salpa mticronata consists of a granular, 

 tolerably viscid yelk, enclosing a large, vesicular germinal vesicle, with a simple germinal 

 spot. No vitelline membrane was to be detected, — the only covering of the ovum being 

 the ovisac, Avhich is closely applied to the surface of the vitellus, and is lined internally 

 with a layer of small nucleated cells. The peduncle of the ovisac is a short, narrow duct, 

 which only becomes a little thicker at its anterior end and, like the ovisac, is lined by an 

 epithelium. Its anterior end opens into the atrium ; and in the vicinity of the aperture 



* Zoologische Untersuchungen, Zweites Heft, Salpen u. Verwandte. Gicssen, 1854. 



