DEVELOPMENT OF HYDRA 



179 



the testes, from a nest of interstitial cells, in the centre of which, distinct 

 from the start, the single ovum lies. In rare cases in H. viridis, 

 H. fusca, and H. grisea there are two ova ; in H. dicecia there may be 

 several. 



Development. — The ovum of Hydra is the successful central cell 

 in the ovary. It is at first amoeboid, and becomes more and more 

 rich at the expense of its neighbours. Their remains (perhaps nuclei) 

 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 



\. ' '.«•' '•;'.••• '-'W' -5 



o 



>rf3 



» » • • » 



• • t , • • • . j 



. x» ••• •-/ ' -x •/ / . . • 



1^>k 



"■+ 



end 





Fig. 89. — Development of Hydra. — After Brauer. 



1. sp.. Spermatozoa. 



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



spherules. 



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



4. Ovum protruding ; w.. the nucleus; ec/., the ruptured ectoderm; 



end., the endoderm. 



5. Section of blastula or blastosphere — Ect., ectoderm ; End., 



endoderm — being formed. 



6. Section of young Hydra. Ect., Ectoderm ; End., endoderm ; 



g.c, gut cavity ; sh., ruptured envelopes. 



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 

 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. 89, 4). 



