102 ELEMENTARY BIOLOGY. [X. 



a. The large and small cells of the ectoderm and 

 its thread-cells, their arrangement and rela- 

 tions. (3.) 



ft. The cells of the endoderm: large, nucleated, 

 with a flattened base and a rounded free end : 

 their arrangement in a single layer. 



7. The thin intermediate layer (muscular stratum) 

 between ectoderm and endoderm. 



S. The body cavity. 



6. Tease out in water a specimen which has been treated 

 with weak chromic acid (0. 1$) 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 (endoderm) cells.] 



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

 coverslip, and examine with a high power. According 

 to its state of maturity the following contents will be 

 found in it 



a. A collection of the smaller ectoderm cells. 



ft. The same but having lost their nucleus and 



become hyaline. 

 7. Cells otherwise like ft. but with a long filament 



proceeding from them. 



. Ripe spermatozoa : bodies consisting of a very 

 small oval head to which a very delicate fila- 

 ment is attached, and which should they get 

 free swim about in the water by the movements 

 of this filament. They may frequently be seen 

 in motion within the unruptured testis. 



