ETDROZOA. 51 



SECTION II. 



DEVELOPMENT OF HYDROZOA. 



The fertilised ovum, in all the Hydrozoa, under- 

 goes yolk-division. This process would seem to 

 be determined by the previous division of the 

 germ-vesicle, which, according to Gegenbaur, in 

 some of these animals at least, does not disappear 

 immediately after fecundation. 



The embryo which results may be developed 

 from the whole, or only a portion, of the Adtellus. 

 It usually appears as a minute, free-swimming 

 ciliated body, but, in some instances, presents a 

 different aspect. The ectoderm and endoderm of 

 the adult Hj^drozoon correspond with the inner 

 and outer layers into which the blastoderm of the 

 embryo soon separates, the cavity which is at the 

 same time formed representing the somatic cavity 

 of the future animal. 



Hydride. — The modification of one end of 

 the body into a hydrorhiza, the formation of a 

 mouth, and of tentacular processes, are the only 

 changes, save those of growth, which seem needed 

 to bring such an embryo into the condition of a 

 perfect Hydra. But observations are yet wan ting- 

 on the development of this organism. The re- 

 searches of Laurent point to the conclusion that, 

 in the production of the young Hydra, a part 

 only of the ovum is directly concerned. 



The polypite thus resulting from a true genera- 

 tive act may subsequently, by gemmation, give 



E 2 



