50 ELEMENTARY ZOOLOGY 



cells become modified and pushed up between the ecto- 

 derm cells to form cnidoblast cells. Many of the 

 endoderm as well as ectoderm cells have muscle-processes 

 which spread out from the base of the cell and which 

 serve to contract and expand the body. 



TECHNICAL NOTE. In the specimens which have been collected 

 perhaps two methods of reproduction will be observed. Place 

 healthy Hydrce in a wide-mouthed jar in the sunlight with plenty 

 of water and food. In a few days active budding will take place. 



Observe the method of reproduction in Hydra. Com- 

 monly the parent produces small buds, which at first are 

 only evaginations of the body- wall, but which later 

 develop tentacles and a mouth of their own. Subse- 

 quently the bud becomes constricted at the base, separates 

 from the parent, and the young Hydra begins a distinct 

 existence. 



Another mode of reproduction takes place which, in 

 distinction from the asexual method just mentioned, is 

 called sexual reproduction. This last is the method 

 common to most of the higher organisms. You may note 

 that in some Hydra there is a swelling or bulging of the 

 ectoderm of the body-wall in the region just below the 

 tentacles. These are the sperm-glands. Within these are 

 produced sperm-cells which break away in great clusters 

 to fertilize the ova, or eggs. Note a larger bulging of the 

 body-wall nearer the lower end of the body which, under 

 high power, has a granular appearance. This is the egg- 

 gland, in which develops a single ovum or egg. The 

 ovum breaks from its covering and is fertilized by sperm- 

 cells from another individual. In forms like Hydra, 

 where both sexes are represented in a single individual, 

 the organism is termed vioiurcunis or hermaphroditic* In 

 connection with reproduction Chapter XIII should be 

 studied. 



