HYDRA AND CCELENTERATES IN GENERAL 137 



FERTILIZATION. Fertilization usually occurs within two hours. 

 Several sperms penetrate the egg membrane, but only one enters 

 the egg itself. If not fertilized within twenty-four hours, the egg 

 becomes sterile. The sperm brings a nucleus containing six 

 chromosomes into the egg. The male and female nuclei unite, 

 forming the fusion nucleus. 



EMBRYOLOGY. Cleavage, which now begins, is total and regu- 

 lar. A well-defined cleavage cavity is present at the end of the 

 third cleavage, i.e. the eight-celled stage. When the blastula is 

 completed, it resembles a hollow sphere with a single layer of 

 epithelial cells composing its wall. These cells may be called the 

 primitive ectoderm. By mitotic division they form entoderm 

 cells which drop into the cleavage cavity, completely filling it. 

 The gastrula, therefore, is a solid sphere of cells differentiated 

 into a single outer layer, the ectoderm and an irregular central 

 mass, the entoderm. The ectoderm surrounds the gastrula with 

 two envelopes. The outer is a thick chitinous shell covered 

 with sharp projections; the inner is a thin gelatinous membrane. 



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 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 ento- 

 derm 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 phenomenon of regenera- 

 tion is appropriate at this place, since the power of animals to 

 restore lost parts was first discovered in Hydra by Trembley 

 in 1740. 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, produce 

 a new individual; that each piece of a Hydra split longitudinally 



