VI.] THE FRESH -WATER POLYPES. 353 



fi. The secretory cells of the endoderm ; deeply-staining 

 pear-shaped cells, interposed between the bases of 

 the larger ones. Numerous 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 nucleus. 



y. The sub-epitheloid layer ; composed of small un- 



differentiated cells among the bases of a and ft. 

 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 

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

 borne upon a single cell. 



As these structures are capable of withdrawal, and 

 usually retracted during digestion 1 , 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; (cf. Sect. 5. e). Their 

 nuclei. Their protoplasm ; differentiated superficially 

 into a finely granulated cuticle. Look especially for 



1 As would appear to be the case with the cilia of the intestinal 

 epithelium of the earthworm (cf. p. 267). 



M. 23 



