HYDRA AND OBELIA. POLYPS AND MEDUSM fji 



pseudopodia, by which it swallows the rest of the interstitial 

 cells contained in the swelling. At the same time it lays up 

 in its protoplasm numerous dark, spherical granules of yolk. 

 As the swelling increases, the musculo-epithelial cells are. 

 stretched, their conical bodies forming long stalks, which 

 are pushed apart by the oocyte, and their outer layer form- 

 ing a thin covering for the latter. When the oocyte has 

 swallowed all the surrounding cells it withdraws its pseudo- 

 podia and becomes a large rounded body, about which a 

 gelatinous coat is secreted. Polar bodies are now formed, 

 the covering of musculo-epithelial cells parts and shrinks 

 back so that the ovum is exposed save for the gelatinous 

 coat, and fertilisation is effected by 

 one of the spermatozoa which are 

 present in the surrounding water. In 

 the formation of a testis the multi- 

 plication of the interstitial cells 

 stretches the musculo-epithelial cells 

 as in the ovary. The interstitial cells 

 become spermatocytes, which lie 

 among the stalks of the musculo- 

 epithelial cells and undergo two 

 divisions as in the frog, the resulting 

 cells developing into spermatozoa 

 with a conical head, a neck, and a 

 tail. By the breaking of the covering 

 ov., Ovary, with nearly ripe layer the spermatozoa are set free 



ovum; te., testes. j • • .1 . , ., 



See also Plate XVI. and SWlm ln the Water > where the y 



perish unless they find a ripe ovum. 

 Since either the ovary or the testis generally ripens first, 

 cross-fertilisation will usually take place, but it does not 

 appear that self-fertilisation is always impossible. 



Fig. 106. — Hydra hang- 

 ing from water- weed. 



After fertilisation the oosperm undergoes cleavage into blastomeres 

 Development 'P" I0 ^' w *" c ^ as '^y increase in numbers form at first a 

 hollow sphere known as the blastula, whose wall consists 

 of a single layer of cells. Some of these migrate into the hollow, 

 which they fill. The outer layer now represents ectoderm and the 

 inner mass endoderm. The cells of the ectoderm become smaller than 

 those of the endoderm and lose their yolk granules. A thick, spiny 

 covering of a horny substance is now secreted by the ectoderm, and the 

 round, prickly body thus formed falls away from the parent and rests 

 for several weeks, during which it may be carried about by currents, in 



12 



