DEVELOPMENT OF HYDRA. 129 



Development. — The ovum bursts the thinned ectoderm, 

 but remains for a while adherent to the parent. At this 

 stage it extrudes polar bodies, and is fertilised. The fixed 

 end may be called the vegetative pole ; the spermatozoon 

 enters at the opposite or animal pole. 



The fertilised ovum undergoes complete segmentation, 

 resulting, according to Kleinenberg, in a solid ball of cells or 

 morula, but acccording to Kerschner, in a blastosphere. 

 From the walls of this blastosphere, the latter observer saw 

 cells migrating inwards, filling up the cavity and forming the 

 future endoderm. This internal immigration from one pole 

 of the blastosphere seems to occur in the development of a 

 number of Coelenterates. A triple protective envelope (chitin- 

 oid, vitelline, and mucous), is formed around the embryo 

 of Hydra viridis, and thus ensheathed the embryo drops 

 from the parent. Kleinenberg said that the superficial 

 ectoderm was sacrificed in making the external chitinoid 

 coat, but Kerschner denies this. 



In the middle of the mass of cells a cavity develops, and 

 the surrounding units are definitely arranged in two layers, — 

 ectoderm and endoderm. The chitinoid layer bursts, and the 

 embryo, still surrounded by the inner envelope, emerges. It 

 elongates, acquires a mouth by rupture at one pole, and buds 

 out tentacles. The inner envelope is lost, and the young 

 Hydra fixes itself, and begins to live as its parents did. It 

 is at the vegetative pole, apparently, that the mouth is formed. 



Forms like Hydra. — Simpler than Hydra is Protohydra, 

 a form without tentacles, occurring both in the sea and in 

 fresh water. A similar fresh-water Microhydra has also been 

 described. A strange animal — Polypodium — whose history 

 is incompletely known, has been described as a parasite on 

 the eggs of sturgeons. 



The class Hydrozoa, illustrated (2) by several Hydro - 

 medusse. 



As Hydra is too simple to be typical of the majority of Hydrozoa, we 

 must take account of those to which the name Hydromedusse is properly 

 restricted. This order includes the hydroid colonies or zoophytes which 

 may be compared to Hydra with many buds ; but linked to these, and 

 often included in their life-history, are medusoid forms which sometimes 

 superficially resemble small jellyfish. 



In many cases we know that these medusoid forms are the liberated 



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