REPRODUCTION 



217 



oval body. Pairing takes place in September and October. The 

 male seizes the female, throws her upon her back, and passes 

 semen through the tubular limbs of his first abdominal segment 

 on to the parts in the neighbourhood of her oviducts, the limbs 

 of his second abdominal pair aiding the process by working 

 to and fro on the hollows of the first. The semen consists of a 

 sticky substance, secreted by the vasa deferentia, carrying the 



■ B 



Fig. 149. — Spermatozoa of a crayfish. 



A, Whole spermatozoon from above ; B, part, enlarged, in section. 

 cps., capsule ; pr., stiff processes. 



spermatozoa, and forms white masses on the sterna of the female. 

 The eggs, which are large and yolky, are laid in November. The 

 processes of the spermatozoa adhere to them, and by a sudden 

 expansion of the contents of the capsule the rest of the body is 

 forced into the ovum. Each egg is attached to one of the hairs 

 on the abdominal Hmbs by a stalked shell formed of a substance 

 secreted by certain glands on the sterna, and is thus under the 

 protection of the mother during its development. By the division 

 of the nucleus of the fertilised ovum a syncytium is formed 

 which does not divide into cells until a number of nuclei have 

 arisen. The young are hatched at the beginning of the next 

 M.z. — 8 



