122 Dr. j\I. Ussow on a new 



of Poly podium which Was not yet developed into a stolon. 

 These were distinguished by their smaller dimensions (0'7 

 millim.), and had the appearance and structure of a planula, 

 although they were destitute of ciliation (which, however, 

 may have been lost after their entrance into the ova of the 

 Sterlet). ISucli an embryo has a large central cavity, which, 

 I suppose, has originated by delaraination. 



The single-layered ectoderm has the same character as in 

 the stolo which is just beginning to bud. The fusiform 

 elements of the muscular layer are not yet recognizable. The 

 single-layered endoderm shows just as little difference from 

 that of the subsequent stage of the stolo, and has the form 

 of a blind sac, the walls of which only touch the ectoderm at 

 one point where they are united with it. 



That this form belongs to the developmental cycle of the 

 Polypodium is shown, on the one hand, by the character of 

 the cells of which it consists, and, on the other, by its being 

 found in the ovum of the Sterlet, surrounded by yolk. As a 

 definitive proof, however, it would be desirable to observe 

 earlier free-living developmental phases of this larva, and to 

 trace their change of form into the elongated sac (stolo) whicli 

 forms the primary and secondary buds. 



It appears to me that from all that has been stated the 

 Coelenteric nature of our animal is sufHciently clear, as also 

 its belonging to the Hydroidea. The very Jong course of 

 development of Polypodium may, in my opinion, be easily 

 explained by the parasitism of the larva (D), which, as is 

 the case in Cunoctantha^ grows into the stolo (A), which, 

 after the formation of the buds a and Z*, divides into the free- 

 living individuals of the generation B. The further division 

 and production of the generations B' and B", and especially 

 the secondary (aB, aB', and aB^) and tertiary (/3 B, /3 B', 

 ft W) generations, may be more difficult to explain, and the 

 more so as we do not know their transformation into the sexual 

 animal (C). 



The whole character of the ecto- and endodermal cells 

 reminds one most of Hydra ; tlie separation of the muscular 

 lamella from the ectoderm it has in common with Myriothela, 



If we mentally imagine the lower part of the body (the 

 primitive foot) of generation B somewhat more elongated, we 

 obtain, as it were, two circlets of tentacles, which are situated 

 close to and at the side of the buccal aperture. The latter, 

 placed at the apex of the proboscis, leads into the primitive 



