REPRODUCTION 



221 



becomes exposed by the breaking of the ectoderm, but it still 

 remains attached by a stalk. At this time the sperm cells are 

 liberated from the testis in large numbers. They swim about 

 in the water and by some means, probably a chemical stimulus 

 originating in the egg, they are attracted to the egg. One of 

 the sperm cells penetrates the protoplasm and fuses with the egg 

 nucleus. This "fertilizing" process initiates the developing 

 process. A membrane is first formed around the egg and by 

 repeated cell division a cylindrical embryo is developed. The 

 membrane then breaks and the ciliated larva is set free at the 



FIG. 124. Longitudinal section of small Turbellarian, Microstomum, which 

 multiplies asexually by strobilation. b, Brain; c, ciliated pit; d, planes of 

 division; e, eye-spot; ent, entoderm; g, intestine; gl, gland cells; m, mouth 

 (original); m f , mouth of second zooid; m 2 , m 3 , mouths of offspring of second and 

 third orders. The strobila consists of a chain of four nearly completed zooids. 

 (From Galloway). 



time when four tentacles are just beginning to develop. After 

 swimming for a time the larva becomes attached and a mouth 

 is formed. From three to five more tentacles appear in the 

 spaces between the others and the young hydra is complete. 

 After maturing a number of ova the parent hydra dies. 



501. Some annelid worms also reproduce by asexual methods, 

 but among the higher forms like nereis and the earthworm repro- 

 duction is wholly by the sexual method. In nereis the sexes 

 are distinct; each individual produces either eggs or sperm, but 

 not both. The reproductive cells are differentiated in size and 

 form, very much as in hydra. They are developed from cells 

 of the mesodermal epithelium lining the body cavity (on the 



