GENERATION AND DEVELOPMENT. 23 



phoridcB, on the other hand, the blastoderm, whence the organs are developed, is, at first, 

 confined to a comparatively small portion of the divided yelk, where it appears as a 

 thickened elevation. What becomes of the rest of the germinal mass, and what organ is 

 first formed in the Plii/sophoridce, is not known, but in the Cal^cophorida, at least in DijjJii/es, 

 Gegenbaur states tliat the first-formed protuberant blastoderm gives rise to a nectocalyx. 

 The short and broad peduncle of this body is traversed by a canal, which expands within the 

 remainder of the germinal mass into a ciliated cavity, which would appear to correspond 

 with the internal cavity of the germ in the other orders. The nectocalyx attains a consider- 

 able relative size, and carries about the germinal mass with its contained cavity as an 

 appendage at its proximal end. The first rudiments of the polypites bud out between this 

 appendage and the nectocalyx (See PI. V). 



Thus far, Gegenbaur's direct observations go. He concludes from them that the 

 nectocalyx which is first formed is the distal one of the two possessed by the adult, 

 and that the mass of the yelk with its central cavity is converted into the somatocyst, 

 which lies within the substance of the proximal nectocalyx of the adult. 



The first conclusion is grounded on the fact that the nectocalycine duct divides into its 

 canals close to the apex of the nectosac, which, as Gegenbaur justly points out, is an arrange- 

 ment characteristic of the distal nectocalyx. But I confess this argument loses much of its 

 apparent weight in my mind, when I reflect on the inmiense changes the minute nectosac 

 must undergo before it can attain its adult form, and on the readiness with which, during this 

 metamorphosis, the relations of the point of division of the duct to the sac might be changed. 



Again, without by any means denying the possible or probable validity of the second 

 conclusion, I must remark that mere similarity of tissue is hardly a sufficient ground 

 for assuming the identity of an embryonic with an adult structure. 



Of the mode of formation of the body of the embryo and of the pneumatocyst in the 

 Phi/sojjJiorida nothing is at present known. The youngest forms hitherto observed are those 

 described by Gegenbaur (in T?liysophora, Forskalia, and Agalmd) and myself (in I'hysalia, infra), 

 and they were already provided with a pneumatocyst of large proportional size, and 

 were terminated by a single large polypite, with a rudimentary tentacle. 



In the Conjnidie, Serfidariadce, and Lucernariadci; the ciliated embryo, after swimming 

 about for awhile, fixes itself by one end, which is modified into the hydrorhiza, and then, 

 elongating, becomes the coenosarc, whence all the appendages are developed. 



A wonderful uniformity pervades the first stages of the development of all the 

 appendages of the Hijdrozoa, notwithstanding their striking ultimate difi'erences ; but before 

 passing to this part of the subject I must direct attention to the laws which govern tlie 

 development of appendages in general upon the coenosarc. 



In the Coryiiida, Sertulariada, and Lucernariadce new polypites are very generally, 

 if not always, developed at, or near, the distal end or surface of the coenosarc, so that 

 the distal polypites are the youngest. Whether this law also holds good with regard to the 

 reproductive organs of the first-named groups is perhaps doubtful. 



Among the Phi/sophorida, on the other hand, a precisely opposite general law obtains; 

 not only the new polypites, but the new nectocalyces and reproductive organs, and 

 even the branches of the tentacles, being developed on the proximal side of the old ones ; 



