172 MANUAL OF ELEMENTARY ZOOLOGY 



mud on the feet of water animals, etc. After a time the ectoderm 

 differentiates into musculo-epithelial and interstitial cells, the jelly is 

 secreted, the shell 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. 



Asexual reproduction also begins with the formation of a 

 swelling of ectoderm by the multiplication of the interstitial 



Fig. 107. — 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 ; »., the nucleus ; ect., the ruptured ectoderm 



of the parent ; end., the endoderm. 



4. Ovum with prickly envelope (sh.) around it. 



5. Section of blastosphere — Ect., ectoderm; End., endoderm — 



being formed. 



6. Section of larva. Ect., ectoderm ; End., endoderm ; g.c, 



enteron ; sh., ruptured envelopes. 



cells, which afterwards become converted into musculo- 

 epithelial and endoderm cells, passing through the structure- 

 less lamella in the latter case. The result of this is an 

 increase in the extent of the ectoderm and endoderm which 

 leads to a bulging of the body-wall. The knob or bud thus 

 formed becomes longer, tentacles grow out around itsfree end, 

 a mouth is formed, and finally the base narrows till the bud 

 breaks free as a new individual, which grows till it reaches 

 the size of the parent. The buds arise in the middle of the 



