302 HYDRA CHAP. 



spermaries and ovaries. Both are found in the same indi- 

 vidual, Hydra being hermaphrodite or monoecious. 



The spermaries (Figs. 73, b, and 74, a, spy) are white 

 conical elevations situated near the distal end of the body : 

 as a rule not more than one or two are present at the same 

 time, but there may be as many as twenty. They are per- 

 fectly colourless, even in the green and brown species, being 

 obviously formed of ectoderm alone. 



In the immature condition the spermary consists of a little 

 heap of interstitial cells covered by an investment of some- 

 what flattened cells formed by a modification of the ordinary 

 large cells of the ectoderm. When mature each of the small 

 internal cells becomes converted into a spertn (p. 194), 

 consisting of a small ovoid head formed from the nucleus of 

 the cell, and of a long vibratile tail formed from its proto- 

 plasm (Fig. 74, g). By the rupture of the investing cells or 

 wall of the spermary the sperms are liberated and swim 

 freely in the water. 



The ovaries (Figs. 73, b, and 74, a, ovy) axe. found 

 nearer the proximal end of the body, and vary in number 

 from one to eight. When ripe an ovary is larger than a 

 spermary, and of a hemispherical form. It begins, like the 

 spermary, as an aggregation of interstitial cells, so that in 

 their earlier stages the sex of the gonads is indeterminate. 

 But while in the spermary each cell is converted into a 

 sperm, in the ovary one cell (Fig. 74, a, ov), soon begins 

 to grow faster than the rest, and becomes amoeboid in 

 form, sending out pseudopods amongst its companions 

 and ingesting the fragments into which they become broken 

 up, thus continually increasing in size at their expense. 

 Ultimately the ovary comes to consist of this single amoeboid 

 ovum and of a layer of superficial cells forming a capsule 

 for it (compare p. 195). As the ovum grows, yolk-granules 



