TYPES OF C(ELENTERA — HYDRA. 



«35 



cells. ' With increase of size the ovum changes its form from 

 amoeboid to cake-like, and from that to spherical. Around the 

 spherical ovum a gelatinous sheath is formed. When the limit of 

 growth is reached, the nucleus or germinal vesicle divides twice in the 

 usual way, and two polar bodies are extruded at the distal pole. There- 

 after the ectoderm of the parent Hydra yields to the increasing strain 

 put upon it, and ruptures, allowing the ovum to protrude. By a broad 

 base it still remains, however, attached to the parent, and in this state 

 it is fertilised, the spermatozoon entering by the distal pole (Fig. 58, 4). 

 The segmentation which follows is total and equal, and results in the 





Fig. 58. — Development of Hydra. — After Brauer. 



germinal vesicle or nucleus ; y.s., yolk 



sp., spermatozoa. 

 ceDoir 



2. AmceDoid ovum ; 



spherules. 



3. Ovum with lobed envelope (s/i) around it. 



4. Ovum protruding ; «, the nucleus ; ect., the ruptured ectoderm ; 



end., the endoderm. 



5. Section of blastosphere — Ect., ectoderm; End., endoderm — 



being formed. 



6. Section of larva. Ect., ectoderm; End., endoderm ; g.c, gut 



cavity ; sh., ruptured envelopes. 



formation of a blastosphere (Fig. 58, 5). By inwandering, or by divi- 

 sion of the cells of the blastosphere, an internal endoderm is formed, 

 and this formation takes place on all sides. In a word, it is multipolar. 

 The segmentation cavity of the blastosphere is thus filled up, and the 

 two layers become differentiated from one another. 



The outer or ectodermic layer forms — (a) an external ' ' chitinoid " 

 shell of several layers ; (b) an internal membrane, homogeneous, thin, 

 and elastic ; and (c) the future ectoderm of the adult. In Hydra fusca 

 the egg is separated from the parent before the shell is formed, and is 

 fastened by its gelatinous sheath to aquatic plants ; in H. viridis and 

 H. grisea the egg falls off after the outer shell has been formed. In 



