162 



GENERAL BIOLOGY 



The hydra is a large and fairly well integrated body of 

 cells, among which division of labor is very obvious. Clear- 

 ly the digestion and absorption of food must fall to the 

 endoderm cells, which alone come into contact with it. 

 Likewise, the protection of the body and responses to stimuli 

 from without must fall to the ectoderm cells. The function 

 of the nettling cells is much more specialized. 



Both spermaries and ovaries develop in the ectoderm, 

 each as a little mass of cells covered by a thin, transparent 

 ectodermal film. 



In the spermary, each included cell develops a motile 

 sperm which when mature may be seen actively swimming 

 about in the transparent conical tip of the spermary of the 

 living hydra. Of the cell mass which is to be the ovary, 

 one cell gains the ascendency and grows at the expense of 

 the others, absorbing their contents and storing up reserve 

 food materials. This cell which when mature is the egg, 

 has the singular amoeboid appearance shown in figure 10 1 b. 

 At maturity the top of the OA r ary 

 bursts, making a passage for the 

 ingress of the sperms, and, in fer- 

 tilization, one of these fuses with 

 the egg cell. 



Study 23. Observations on the 

 structure of the hydra. 



Materials needed: Living hy- 

 dras of any species; budding 

 individuals, and also others bear- 

 ing the sex organs. Mounted 

 FIG. 102. Nettling ceils of the cross sections of the body, and 

 E!ed. ' charged: b> dis ' dissociation preparations of ecto- 

 derm cells. 



Study these things in the order mentioned, using the 

 foregoing account of the hydra as a basis of observations. 



