VI.] THE FRESH-WATER POLYPES. 353 



Occasionally some of the vacuoles may be seen to 

 contain groups of minute needle-shaped crystals. 



y. The secretory cells of the endoderm ; smaller deeply- 

 staining pear-shaped cells, interposed between the 

 bases of the larger ones. Numerous and well-de- 

 fined in sections across the hypostome, fewer in 

 those across the body. They may be recognized 

 by their tapering inner ends, the highly-granular 

 nature of their cell-protoplasm, and by their small 

 deep-staining nucleus. 



g. Place a Hydra in i per cent, osmic acid solution for 

 24 hours, and then tease up a portion of the en- 

 doderm in weak glycerine, under a low power. Ex- 

 amine, under your highest objective, and look for the 

 Jlagella ; long whip-lash filaments, one to six being 

 borne upon a single cell. 



As these structures are capable of withdrawal, cells 

 will be present which do not bear them. Look for 

 specimens showing stages in their elongation, 



6. Structural analysis of the individual cells. Pre- 

 serve some Hydras in Miiller's fluid (2 — 3 days). Transfer 

 to alcohol of increasing strengths, and finally tease up under 

 a low power in eosin or haematoxylin solution. Alter- 

 natively, preserve for a similar period in i per cent, solu- 

 tion of ammonia bichromate, transfer to alcohol and tease 

 up in carmine solution. Select the best preserved cells and 

 examine, under your highest power, in order — 



a. The larger ectoderm cells ; nucleated, with a flattened 

 base and a rounded free end (cf. Sect. 5. e). Their 

 nuclei ; frequently containing two nucleoli. Their pro- 

 toplasm j often differentiated into a superficial cuticle- 

 like product. TvOok especially for Kleifienberg' s fibres 



M. 23 



