DEVELOPM ENT 



lOI 



cracks, and the embryo projects. A split in the endoderm forms 

 the enteron, tentacles grow out, a mouth is formed, and finally 

 the young hydra frees itself from the remains of the shell, moves 

 away, and begins to feed and grow (Fig. 66). 



Asexual reproduction also begins with the formation of a 

 swelling of ectoderm by the multiplication of the interstitial 

 cells, which afterwards become converted into musculo-epithelial 

 and endoderm cells, passing through the structureless lamella in 

 the latter case. The result of this is an increase in the extent of the 



Bad 





\:^- End V^:^ 



Fig. 66. — The development of Hydra. — After Brauer. 



1. sp.. Spermatozoa. 



2. AmcEboid ovum ; g.v., germinal vesicle or nucleus ; y.s., yolk spherules. 



3. Ovum protruding ; n., its nucleus ; ect., the ruptured ectoderm of the parent ; end., the endoderm. 



4. Prickly envelope (sh.) of embryo liberated from parent. 



5. Section of blastula — Ect., ectoderm ; End., endoderm arising ; it will fill the blastula cavity (blastocoele). 



6. Section of young Hydra leaving shell. Ect., ectoderm ; End., endoderm ; g.c, enteron ; sh., ruptured 



envelopes. 



ectoderm and endoderm which leads to a bulging of the body 

 wall. The knob or bud thus formed becomes larger, about four 

 or five tentacles grow out around its free end, a mouth is formed, 

 and finally the base narrows. Then the bud's tentacles attach 

 themselves to a soHd object, and pull the young creature apart 

 from its parent. It grows in size and adds a few more tentacles. 

 The buds arise in the middle or lower part of the body of the 

 parent. Several may be formed at the same time, and a bud may 

 form secondary buds before it is set free. While it is still on the 

 parent, the bud is wholly a part of the body of the latter. Each of 

 the layers of the parent is continuous with the corresponding layer 

 of the bud, a suitable stimulus is transmitted by the nervous 

 system from one to the other, and the entera are in free 



