148 



PHYLUM C(ELENTERA. 



accumulate within the ovum as "yolk spherules" or "pseudo-cells." 

 Some yolk-granules, formed within the ovum, may coalesce in " pseudo- 

 cells" of another type. 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, and 

 two polar bodies are extruded at the distal pole. There are twelve 

 chromosomes to begin with, and by the reduction division in forming the 

 first polar body, the number is reduced to six. Thereafter the ectoderm 

 of the parent Hydra yields to the increasing strain put upon it, and 



:: . 



2 O 



FlG. 70. Development of Hydra. After Brauer. 



I. sfl. t spermatozoa. 



9. Amoeboid ovum ; g.v., germinal vesicle or nucleus ; y.s., yolk 

 spherules. 



3. Ovum with lobed envelope (sh.) around it. 



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



end., the endoderm. 



5. Section of blastosphere Ret., ectoderm ; End., endoderm 



being formed. 



6. Section of larva. Ect., ectoderm J End., endoderm ; ".., gut 



cavity : sh., ruptured envelopes. 



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. 70, 4). 



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

 the formation pf a blastosphere (Fig. 70, 5). By in wandering, or by 

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

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

 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, 



