100 



HYDRA. PHYLUM CCELENTERATA 



In the ovary one of the interstitial cells becomes an oocyte. 

 This increases in size and begins to throw out 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 

 nunieious 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, 

 their outer layer forming a thin covering for the latter. When 

 the oocyte has swallowed all the surrounding cells it withdraws 

 its pseudopodia and becomes a large rounded body, about which a 

 gelatinous coat is secreted. Polar bodies (p. 620) are now formed, 

 the covering of musculo-epithehal cells parts and shrinks back so 

 that the ovum is exposed save for the gelatinous coat, and fertilisa- 

 tion 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, the resulting cells developing into sperma- 

 tozoa with a conical head, a neck, and a tail. By the breaking of 

 the covering layer the spermatozoa are set free and swim in the 

 water, where they perish unless they find a ripe ovum. Since in 

 the green hydra the testis generally ripens first, cross-fertiHsation 

 will usually take place, but it does not appear that self-fertilisation 

 is always impossible in this species. 



DEVELOPMENT 



After fertilisation the egg undergoes cleavage into blasto- 

 meres (p. 621), which as they 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 ecto- 

 derm, 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 mud on the feet of water animals, 

 etc. After a time the ectoderm differentiates into musculo- 

 epithelial and interstitial cells, the mesogloea is secreted, the shell 



