1 64 ORGANIZATION OF THE HEAD v. 13- 



The testes are paired and sperms are collected at their front ends 

 by vasa efferentia leading into the anterior or reproductive portion of 

 the mesonephros. This consists of a much coiled, thick-walled, vas 

 deferens, whose glands produce material that aggregates the sperms 

 into spermatophores. The vas expands into a broader ampulla 

 (seminal vesicle), which at its lower end gives off a forwardly directed 

 blind diverticulum, the sperm sac, developmentally the lower end of 

 the Miillerian duct, reduced of course in the male; small funnels are 

 still visible at the upper end. 



Transmission of the sperms is produced by a large and complicated 

 pair of claspers. These are modified parts of the pelvic fins of the 

 male, developed into scroll-like organs and containing a pumping 

 mechanism and erectile tissue; they are inserted into the female 

 cloaca. The mechanism of erection is operated by nerves and may 

 involve the liberation of adrenaline; experimental injection of that 

 substance will produce erection, and it is perhaps significant that the 

 male possesses a reserve of adrenaline-producing tissue (see p. 167). 



Development of elasmobranchs is by partial cleavage, producing a 

 blastoderm, perched on the top of a large mass of yolk. The egg is 

 protected by an elaborate egg-case, the 'mermaid's purse', within 

 which development proceeds until the yolk has been used up. 



In several elasmobranchs development is viviparous, the oviduct 

 forming a 'uterus'. In Mustelus there is a yolk-sac placenta, but in 

 Trygon 'uterine milk' is secreted into the embryo by villi (trophone- 

 mata) inserted through the spiracles. 



14. Endocrine glands of elasmobranchs 



Elasmobranch fishes possess the full complement of endocrine 

 glands, but these show some interesting differences from those of 

 higher vertebrates. The pituitary body lies in the usual place below 

 the diencephalon and anterior, intermediate, tuberal, and neural 

 divisions can be recognized. Little is known of the functions of the 

 gland. The gonads of the dogfish retrogress after removal of the pitui- 

 tary. Little or no vasopressin or oxytocin is present. The neuro- 

 intermediate lobe contains a substance that produces the expansion 

 of melanophores. Hogben and Waring have also produced evidence 

 that the pars anterior produces a substance causing contraction of the 

 melanophores, but this has not yet been isolated and the evidence for 

 its existence is indirect. 



The thyroid is formed by a downgrowth from the floor of the 

 pharynx, to which it often remains attached by a narrow stalk con- 



