134 AN INTRODUCTION TO ZOOLOGY 



connected with that of the parent. When full grown, the bud 

 becomes detached and leads a separate existence. The details 

 of the process are briefly as follows. The interstitial cells in a 

 certain region increase in number and volume, producing a slight 

 outbulging of the ectoderm. The growing region is located at 

 the point where the edges of the protrusion meet the body wall. 

 Here the cells are well fed and multiply actively. The ectoderm 

 and entoderm cells of the parent give rise to the corresponding 

 cells of the bud. When the bud is fully grown, the ectoderm cells 

 at its proximal end secrete a sticky substance which is used later 

 for its attachment. The entoderm cells in the same region then 

 unite, separating the cavity of the bud from that of the parent. 

 Finally, the bud becomes detached. The food supply determines 

 the rate of growth of the bud, and a bud may be entirely absorbed 

 by a starving animal (120). 



SEXUAL REPRODUCTION. Whether or not there are definite 

 germ cells in the adult Hydra is still open to question. So far 

 as is known, both ova and spermatozoa arise from indifferent 

 interstitial cells. 



SPERMATOGENESIS. The male cells of Hydra are formed in 

 little conical elevations called testes which project from the surface 

 of the body (Fig. 54, y. t.,m.t.). The testis arises within the ecto- 

 derm from interstitial cells. A single interstitial cell divides 

 mitotically; then adjacent interstitial cells also divide, multi- 

 plication continuing until the ectoderm becomes distended. An 

 indefinite number of long multinucleated cysts (Fig. 62, A) are 

 formed within the testis, each cyst being the product of a single 

 or several interstitial cells. Each interstitial cell is a primordial 

 germ cell ; it gives rise by mitosis to a variable number of sper- 

 matogonia, which contain the somatic number of chromosomes, 

 twelve. Reduction in the number of chromosomes to six occurs 

 just after the spermatogonia have divided to form the primary 

 spermatocytes (Fig. 62 , A, b). The latter give rise to secondary 

 spermatocytes (c) which divide at once, producing spermatids 

 (c). These two spermatocyte divisions take place without the 



