116 



COLLEGE ZOOLOGY 



Mesoglea 



Epidermis 

 of parent 



Mature egg 



Young hydra emerging 



Figure 56. Stages in the development of the hydra. 



whicli drop into the cleavage cavity, com- 

 ]5lctely filling it. The early gastrula, there- 

 fore, is a solid sphere of cells differentiated 

 into a single outer layer, the ectoderm, and 

 an irregular central mass, the endoderm 

 (Fig. 56). The ectoderm secretes two en- 

 velopes around the gastrula; the outer is a 

 thick chitinous shell which may be covered 

 with sharp projections; the inner is a thin 

 gelatinous membrane. Different species of 

 hydras can be identified by the structure of 

 their shells. 



Hatching. The embryo in this condition 

 separates from the parent and falls to the 

 bottom, where it remains unchanged for 

 several weeks. Then interstitial cells make 

 their appearance. A subsequent resting pe- 

 riod is followed by the breaking away of 

 the outer chitinous envelope and the elonga- 

 tion of the escaped embryo (larva). Meso- 

 glea is now secreted between the ectoderm 

 and endoderm cells, and these layers dif- 

 ferentiate to form the adult epithelial tis- 



sues: the epidermis and gastrodermis. A 

 circlet of tentacles arises at one end and a 

 mouth appears in their midst. The young 

 hydra thus formed grows into the adult 

 condition. 



Regeneration 



An account of the phenomena of regen- 

 eration is appropriate at this place, since the 

 power of animals to restore lost parts was 

 first discovered by Trembley in 1740 in the 

 hydra. This investigator found that if hydras 

 were cut into 2, 3, or 4 pieces, each part 

 would grow into an entire animal. Other 

 experimental results obtained by Trembley 

 are that if a hydra is split longitudinally into 

 2 or 4 parts, each part becomes a perfect 

 polyp; that when the head end is split in 

 two, and the parts are separated slightly, a 

 "two-headed" animal results; and that a 

 specimen when turned inside out is able to 

 readjust itself to these new conditions forced 



