94 ORGANISMS OF TISSUES 



(Fig. 38, A). The ovum grows to a relatively large size and be- 

 comes loaded with yolk bodies; the external cells of the ovary 

 finally break away leaving the ovum exposed. A spermatozoon 

 bores its way into the egg and fertilizes it. 



Development begins while the egg is still attached to the 

 parent Hydra. A hollow ball of cells, or blastula, is formed 

 consisting of a single layer of cells enclosing the segmentation 

 cavity (Fig. 38, B). Cells then begin to migrate from the per- 

 iphery into the segmentation cavity which ultimately is filled 

 by the mass of hypoblast cells thus forming a solid endoderm 

 (Fig. 38, C, D). The outer layer of cells, ectoblast, secretes a 

 horny protective envelope about the embryo which now leaves 

 the parent and rests without further development for some 

 time on the bottom of the pond. Finally development begins 

 again; the cyst or shell is ruptured and the embryo escapes. 

 The endoderm cells begin to separate from one another leaving 

 a hollow in the middle; this is the enteric cavity from which 

 a mouth breaks through and tentacles develop around it (Fig. 

 38, F). The gastrula stage or two-layered pouch with blasto- 

 pore is attained with the formation of the mouth, but the 

 method of gastrula tion differs from that described on p. 77. 

 The same end result is reached, in typical development by 

 invagination, here by inwandering or immigration of ectoblastic 

 cells. 



'SYMBIOSIS 



In addition to Microhydra (a form without tentacles) and 

 Hydra fusca (brown Hydra) there is another and an equally 

 common species of fresh water forms Hydra mridis. As the 

 specific name indicates this Hydra is colored green, the color 

 being due to the presence of minute unicellular plants, Chlorella 

 vulgaris. The plant cells are situated in the basal parts of the 

 nutritive muscle cells where they form an almost complete 

 sheath of investing plant organisms, the cells of the ends of the 

 tentacles alone, being free from them. Buds, when formed, 

 include the accompanying plants and are quite as green as the 

 parent. 



