154 



TEXTBOOK OF ZOOLOGY 



rial accumulates at some point near the middle of the column. The 

 bud first appears as a slight superficial bulge. The cell division at 

 this point is very rapid, involving considerable activity in inter- 

 stitial cells. This enlargement rapidly increases in size to form a 

 stalk. An extension of the eiateron extends into the bud, which is 

 essentially an outgrowth of the body wall. Tentacles appear as 

 outpushings of ectoderm and endoderm, and in the terminal posi- 

 tion a mouth is developed. After the bud has attained some size, a 

 constriction occurs between it and the parent. This closes the 

 enteron between bud and parent, and the bud finally separates to 

 become a free individual. 



Embrvo 



?6md.l 



tiydra.- <§exua.l l^production 



Fig. 74. — Methods of reproduction in hydra. (Courtesy General Biological Supply 



House.) 



Sexual Reproduction. — During the summer and fall particularly, 

 hydra reproduces sexually. This involves the production, matura- 

 tion, and union of germ cells. Testes may appear first and ovaries 

 later on the same individual or both gonads may be present at the 

 same time in which case self-fertilization is possible. As a rule, 

 these animals are hermaphroditic or monoecious as suggested before, 

 but it has been reported that individuals of separate sex (dioecious) 

 have been found. The germ cells or gametes develop from inter- 

 stitial cells which accumulate at a certain place between the ectoderm 

 and endoderm, where they multiply by division to form oogonia in 



