in other Hydroids it can have no general significance for the 

 development of the order." 



The remaining history of Hydra is soon told. In this proto- 

 plasmic germ -mass there is formed a small excentric cavity; this 

 is the beginning of the body-cavity, which finally forms a closed 

 sac. Allman remarks on this discovery of Kleinenberg's that it 

 is <jlear that the formation of a body cavity by invagination of the 

 walls [/. e., ectoderm] with the significance which Kowalevsky 

 has assigned to it in other animals, does not exist in Hydra, and 

 just as little will it be found in any other hydroid." It will be 

 seen farther on that in certain medusae, Kowalevsky has discov- 

 ered that the digestive cavity is formed by the invagination of the 

 ectoderm, and we have seen (p. 107) that Metznikorf declares 

 that the ciliated cells lining the gastro-vascular cavity in the embryo 

 of the sponge are the originally external ciliated cells of the pla- 

 nula withdrawn into and lining the body cavity. 



After several weeks the germ bursts the hard shell and escapes 

 into the surrounding water, hut is still surrounded by a thin inner 

 shell. After this a clear superficial zone appears, and a darker 

 one beneath, which is the first indication of the splitting of the 



The embryo soon stretches itself out, a star-shapr s eh it .4. 

 which forms the mouth. The tentacles next appear. The a 

 now bursts open the thin inner shell, and the young Hydra aj 



There is. then, no metamorphosis in the Hydra; no ciliate 

 nula. The adult form is thus reached by a continuous grow 



It will be seen, to anticipate somewhat, that the Hydr 

 actly as in the vertebrates, including man, arises from a 

 developed from a true ovary, which is fertilized by a true 

 spermatic particle ; that the egg passes through a morula i 

 that the germ consists of two germinal layers, while fro 

 outer layer, as probably in the vertebrates, an intermedi; 



logue of the middle germ-lamella of the ve 

 to have originally split off from the ectode 

 regards the chitinous shell of the germ of Hyd 

 of the epidermis of vertebrates, being a pr< 

 or-an in Hydra, but permanent in vertebrates, 



