PHYSIOLOGY OF THE EMBRYO 323 



nerve centres which maintain the vegetative functions of the 

 body. 



Little need to be said concerning the physiological devel- 

 opment of the sense organs. The only sensations which 

 can come into account in the foetal life are the tactile, pain, 

 and, perhaps, some organ sensations. These evidently call 

 forth the movements of the fcetus. 



4. DIFFERENTIATION OF SEXES 



The reproductive organs are developed as follows: On 

 the ventral side of the pronephros, the genital ridge and a 

 special duct, Mailer's duct, running parallel with the 

 Wolffian duct and also opening into the cloaca, are formed. 

 In the male the genital ridge forms the testis, the pronephros 

 forms the hydatid of the epididymus, the Wolffian ducts the 

 vas deferens; the duct of Mailer is obliterated except a small 

 rudiment, called the uterus masculinus. In the female the 

 genital ridge becomes the ovary, Mailer's duct the oviduct; 

 and the mouth of the Mailer's duct at the cloaca dilates and 

 forms the uterus. The Wolffian duct disappears. Nothing 

 is known concerning the causes of sexual differentiation. 



