88 ORGANISMS OF TISSUES 



disc. They are fine thread-like cells crowded in between the 

 epithelial cells, and run out at the basal ends into branching 

 fibers which connect the sensory cells with nerve and muscle 

 cells. 



5. Reproductive Cells. Hydra is hermaphrodite, that is, pro- 

 vided with both male (spermatozoa) and female (ova) germ 

 cells. The former are aggregated into gonads called testes, 

 the latter into gonads called ovaries. When immature, the 

 germ cells cannot be distinguished from other formative cells, 

 but when the organism is mature, the germ cells accumulate 

 between the epithelial cells, forming characteristic swellings 

 of the male and female gonads. The male gonads, which are 

 usually multiple, develop as a rule before the latter, and usually 

 in the vicinity of the tentacle bases, while the ovaries usually 

 form near the foot (Fig. 34). A mature testis contains multi- 

 tudes of spermatozoa which may be seen in active movement 

 within the gonad; but only one egg develops in the ovary. 



6. Formative Cells. The formative or interstitial cells, finally, 

 are present as minute rounded cells heaped up between the 

 epithelial cells on the supporting lamella, and are especially 

 numerous on the hypostome. They form the cell reserves of 

 Hydra, replacing nettle cells and nerve cells when exhausted, 

 and give rise by growth and division to the reproductive cells. 

 During regeneration after injury the new epithelio-muscle cells 

 are likewise derived from these reserves. They are, therefore, 

 typical embryonic or generalized cells from which all other 

 elements of Hydra may be replaced. 



B. THE ENDODERM. The endoderm of Hydra, like the ecto- 

 derm, is made up of six types of cells: (i) nutritive muscle cells; 

 (2) slime cells; (3) albumen cells; (4) sensory cells; (5) nerve cells; 

 and (6) formative cells. The nerve and formative cells, as in 

 the ectoderm, lie between the bases of the epithelial nutritive 

 cells, and are comparatively rare. The slime and sensory cells 

 are almost exclusively limited to the mouth region. 



i. The Nutritive Muscle Cells. These are elongated cylindri- 

 cal cells somewhat enlarged at the distal rounded ends which 

 may bear flagella (Fig. 36). The cytoplasm is highly vacuo- 



