PHYLUM COELENTERATA 



155 



the female gonad and spermatogonia in the male. All phases of 

 maturation (gametogenesis) may be obsei-^^ed in the testis and ovary. 

 The testis produces large numbers of motile spermatozoa, which when 

 mature emerge periodically from an opening in the tip of the testis 

 and are discharged free in the water. In the ovary a single egg 

 develops at the expense of the other oogonia, which are engulfed 

 bodily and used for food. This one cell grows rapidly, and when 



Fig. 75. — Development of hydra. 1, Fertilized ovum; Z, two-cell stage; S, 

 blastula stage; i gastrula, showing ectocferm (ec) and endoderm (en) ; cc, cleavage 

 cavity (blastocoele) ; m, cyst; p.b., polar bodies. (After Tanreuther, Biological 

 Bulletin, Vol. 14.) 



mature it fills the ovary. Fertilization is accomplished by the en- 

 trance of spermatozoa through a rupture in the overlying ectoderm 

 and cross-fertilization usually prevails. A single sperm unites with 

 the mature ovum, and this zygote undergoes the total and equal 

 divisions of cleavage here in place. The process continues until a 

 hollow llastula of many cells is formed. Then follows the formation 

 of the gastrula by a shedding of cells into the cavity (blastocoele) 

 from the inside of the original layer of cells. These new cells on the 

 inside become organized as an endoderm layer, while the original 



