MEANS AND METHODS OF REPRODUCTION 



155 



striction across the middle of the body, the result in either case 

 producing eventually two complete but smaller animals (Fig. 99). 

 Budding in Hydra starts with the formation of a bulge in the 

 body wall which takes on a cylindrical form with a circlet of 

 tentacles developing at the free end of the cylinder. When 

 fully grown, the bud is released as a new, free individual (Fig. 



219). Euplanaria is another com- 

 mon laboratory animal in which 

 fission occurs (Fig. 99). 



Gamic Reproduction. — Gamic 

 or sexual reproduction is repro- 



M-4 



Fig. 100. Fig. 101. 



Fig. 100. — Section of mature ovum, dn, degenerating nuclei of interstitial 

 cells contributing to formation of ovum; e, ectoderm; i, interstitial cells; m, 

 mesoglea; n, nucleus of ovum; o, ovum. (After Tannreuther.) 



Fig. 101. — Section of ovary of Hydra, showing developing ovum. 



duction from an egg whether fertilized or not. It occurs in all 

 metazoans, even in those that reproduce by fission or budding. 

 Thus in Hydra, the gonads, which are not present at all times, 

 develop as localized proliferations of the interstitial cells of the 

 ectoderm. The spermaries or testes, containing numerous sper- 

 matozoa, form as round protuberances near the oral end, while 

 the ovaries, somewhat similar in shape, are formed near the aboral 

 end (Fig. 221). Within each ovary a single cell enlarges to form 

 an egg by the rather unusual method of engulfing surrounding 

 interstitial cells (Figs. 100 and 101). Thus a single cell becomes 

 an egg at the expense of neighboring cells. Hydra is hermaph- 

 roditic, but since the ovary and testis of the same animal 



