324 HYDRA CHAP. 



currents in the tentacles, which are produced by long vibratile flagella 

 present on many of the endoderm-cells. 



8. The thin transparent supporting lamella. Sketch. 



9. Treat a specimen with methyl-green. A slight pressure on the 

 cover-glass will crush the animal, and render the interstitial cells and 

 thread-cells especially distinct. Note also other isolated cells of the 

 ectoderm and endoderm. Sketch. 



10. Examine a specimen with buds in different stages of development, 

 and note as much as possible of the mode of asexual reproduction by 

 gemmation. Sketch. 



11. If none of your specimens bears sexual organs, try to procure a 

 mounted preparation which shows them, and examine first with the 

 low, and then the high power. Note (a) The spermaries several 

 conical swellings, near the bases of the tentacles. They are covered 

 with large ectoderm cells, and contain numerous interstitial cells, each 

 of which eventually gives rise to a sperm with a "head" and vibratile 

 "tail." These are discharged at the apex of the cone, which when 

 ripe may be ruptured by a slight pressure on the cover-glass, (b] The 

 ovaries (sometimes only one), generally situated near the proximal end 

 of the body. They are larger than the spermaries and more spherical, 

 but at first have a similar structure. When ripe a single ovum is found 

 in each. Sketch. 



Place some Hydne in a watch-glass with a very small amount of 

 water, and when they have expanded, pour quickly over them a warm 

 saturated solution of corrosive sublimate in alcohol or water. Wash 

 several times with weak alcohol, stain for a few minutes with borax- 

 carmine, or haematoxylin, and wash with weak and then with stronger 

 alcohol. Place in absolute alcohol for a few minutes, and afterwards in 

 turpentine or oil of cloves ; mount in balsam. Work through 5-8 

 again, noting especially the characters of the various cells and their 

 nuclei, as w r ell as 



12. The contractile processes coming off from the inner ends of the 

 large ectoderm-cells (Fig. 76, B). These extend longitudinally, and lie 

 against the outer surface of the supporting lamella. Sketch. 



Examine transverse sections through the body or tentacles (Fig. 76, B) 

 prepared as directed on p. 136, after killing and fixing the specimens as 

 above. Work through 6-8 again, noting the various cells and their 

 nuclei, &c. Observe especially 



