THE HYDRA AS A SIMPLE METAZOAN 



271 



may appear together. Thus the hydra is an hermaphroditic 

 animal, or one producing both male and female cells in the same 

 individual. If the early stages of a testis are examined it is found 

 that a group of interstitial cells have multiplied to form the early 

 germ cells, from which the spermatozoa, or male germ cells are 



Fig. 131. — Formation of germ cells of hydra. 



1, part of longitudinal eection {cf. Fig. 132 A), showing two oocyte? (o and h) in ectoderm. 

 2 to 4, sections through one of ttie compartmenis of the teste? showing stages in develop- 

 ment of interstitial cells into spermatozoa (a, h, c, d). 5 to 10, formation of spermatozoa. 

 11 and 12, optical sections of compartments of testes (c/. 2 to 4) in two different stages 

 immediately after removal from living hydra. (After Tannreuther.) 



differentiated (Fig. 131). When the spermatozoa are fully 

 matured, they may be seen moving actively within the testis. 

 They are discharged into the water by the opening of the apex of 

 the testis, which thus liberates successive swarms of sperm. 

 They swim about until they perish or come in contact with a ripe 

 ovum, which is entered in fertilization. By far the greater 



