REPRODUCTION 



221 



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

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

 Hberated from the testis in large numbers. They swim abt)ut 

 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 ciHated 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', mouth of second zooid; m-, m^, 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 comi)lete. 

 After maturing a number of ova the parent hydra dies. 



501. Some annehd worms also reproduce by asexual methcKls. 

 but among the higher forms Hke nereis and the earthworm re|)ro- 

 duction is wholly by the sexual method. In nereis the se.xcs 

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

 not both. The reproductive cells are dilTerentiated in si/e ami 

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

 of the mesodermal epithelium lining the body cavity [on the 



