HYDRA AND CCELENTERATES IN GENERAL 



formation of cell walls, i.e. each primary spermatocyie develops 

 into a four-nucleated cell which represents the four spermatids. 

 Within this cell the spermatids transform into spermatozoa 

 (Fig. 62, B). A single cyst may contain 

 representatives of all of these cell gener- 

 ations spermatogonia, primary sper- 

 matocytes, secondary spermatocytes, 

 spermatids, and spermatozoa. The 

 mature spermatozoa break out of the 

 vesicle in which they are formed, and 

 swim about in the distal end of the cyst 

 (Fig. 62, A, d); they finally reach the 

 outside by way of a minute temporary 

 opening in the end of the cyst. The 

 mature spermatozoa swim about in the 

 water searching for an egg; their activ- 

 ity continues from one to three days. 



OOGENESIS. The egg is first dis- 

 tinguished from the interstitial cells of 

 the ectoderm by its slightly greater size, 

 its spherical shape, and the compara- 

 tively large volume of its nucleus. As 

 the eggs grow in size the neighboring 

 interstitial cells increase in number by 

 mitosis, and also become larger. The 

 whole structure may at this time be 



called an ovary (Figs. 63, A; 54, y. e.). FlG - 62 ' Parts of a testis of 



Hydra. A, a single cyst 



showing spermatogo- 

 nia, primary spermato- 



The nourishment of the egg is at first 

 similar to that of the other ectoderm 

 cells, but later the interstitial cells near 

 it are engulfed, their contents becoming 

 part of the ovum. The nuclei of these 

 interstitial cells furnish the yolk of the 

 growing egg. Usually only one egg is 

 developed in a single ovary, but some- 



cytes (b), secondary 

 spermatocytes and 

 spermatids (r), and 

 spermatozoa (d) ; B, 

 developing spermato- 

 zoa. (After Tann- 

 reuther in Biol. Bui.) 



