PHYLUM C(ELENTERATA 



117 



cells entoderm. The ectoderm now secretes a thick chitinous 

 shell covered with sharp projections. The embryo then separates 

 from the parent and falls to the bottom, where it remains un- 

 changed for several weeks. Then interstitial cells make their 

 appearance. A subsequent resting period is followed by the 

 breaking away of the outer chitinous envelope and the elongation 

 of the escaped embryo. 

 Mesoglea is now secreted 

 by the ectoderm and 

 entoderm cells; a circlet 

 of tentacles arises at 

 one end, and a mouth 

 appears in their midst. 

 The young Hydra thus 

 formed soon grows into 

 the adult condition. 



Regeneration. — An 

 account of the phe- 

 nomenon of regenera- 

 tion IS appropriate at Fig. 72. — Regeneration and grafting in 

 this Dlace since the Uydra. A, seven-headed Hydra made by split- 

 ' . ting distal ends lengthwise. B, a piece of Hydra 



power OI animals to regenerating an entire animal. C, part of one 

 restore lost parts was H y dra grafted upon another. (From Morgan, 



A, after Trembley ; B, after Morgan; C, after 



first discovered in Hydra King.) 

 by Trembley in 1744. 



This investigator found that if Hydras were cut into two, 

 three, or four pieces, each part would grow into an entire 

 animal. Other experimental results obtained by Trembley are 

 that the hypostome, together with the tentacles, if cut off, may 

 produce a new individual ; that each piece of a Hydra split longi- 

 tudinally into two or four parts, becomes a perfect polyp, and 

 that when the head end is split in two and the parts separated 

 slightly, a two-headed animal results (Fig. 72, A). 



Regeneration may be defined as the replacing of an entire 

 organism by a part of the same. It takes piace not only in 



