96 ORGANISMS OF TISSUES 



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 pe- 

 riphery 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 gastrulation differs from that described on p. 80. 

 The same result is reached, in typical development by in- 

 vagination, here by inwandering or immigration of ectoblastic 

 cells. 



SYMBIOSIS 



I 



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 viridis. 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. 



