x.] THE FRESH-WATER POLYPES. 105 



J3. The cells of the endoderm : large, nucleated, with 

 a flattened base and a rounded free end : their 

 arrangement in a single layer. 



y. The thin intermediate layer (muscular stratum) 

 between ectoderm and endoderm. 



8. The body cavity. 



6. Tease out in water a specimen which has been 

 treated with weak chromic acid (o. i) or with osmic 

 acid : make out the various cells already described : 

 notice branched tails proceeding from the narrower 

 ends of the larger ectoderm cells. 



[7. Tease out a fresh Hydra in water and observe the 

 various cells. Note the amoeboid movements exhibited 

 by some, and the single cilium attached to other (endo- 

 derm) cells.] 



8. Gently flatten out a testis in water by pressure on the 

 coverslip, and examine with a high power. Accord- 

 ing to its state of maturity the following contents will 

 be found in it 



a. A collection of the smaller ectoderm cells. 



/?. The same but having lost their nucleus and 

 become hyaline. 



y. Cells otherwise like /?, but with a long filament 

 proceeding from them. 



e>. Ripe spermatozoa: bodies consisting of a very 

 small oval head to which a very delicate filament 

 is attached, and which, should they get free, swim 

 about in the water by the movements of this fila- 

 ment. They may frequently be seen in motion 

 within the unruptured testis. 



