696 MANUAL OF PHYSIOLOGY. 



this into an alimentary or anal portion, and an anterior or urinary 

 portion. The septum, dividing the urogenitary from the alimen- 

 tary portion of the cloaca, forms, externally, the perinseum. 



At the aperture of the cloaca an eminence arises which develops 

 into the penis in the male, the clitoris in the female. Around 

 this eminence is a fold of integuments, which forms the labia in 

 the female, the scrotum in the male. 



In the female this integumentary covering enlarges much more 

 than the clitoris, and covers it in, the urethral orifice opening 

 just below the clitoris. 



In the male the urethral orifice at first opens at the base of the 

 penis, but eventually a groove is formed on the under surface of 

 this organ, which becomes converted into a canal, and forms the 

 urethra. 



BLOOD- VASCULAR SYSTEM. 



In the mammalian embryo this may be appropriately divided 

 into two systems of different dates ; the first, or early circulation, 

 which is confined to the yolk sac ; and the second, or later circu- 

 lation, which passes through the placenta. 



The Primitive Heart arises from the splanchnopleural layer of 

 the mesoblast, just at the point where this forms the under wall 

 of the forepart of the alimentary canal. When the formation of 

 the folds of the embryo was described, it was stated that the groove 

 of the cephalic fold tended to grow backwards towards the tail 

 end of the embryo. This groove is limited behind by the somato- 

 pleural layer of the raesoblast, and posteriorly to this is a cavity 

 formed by the cleavage of the mesoblast, called the pleuroperi- 

 toneal cavity. In the early stages of development, the posterior 

 wall of this small cavity is formed by the splauchnopleural layer 

 of the mesoblast. The heart arises at the point at which the 

 splanchnopleure tends to travel forwards to meet the uncleft me- 

 soblast, and thus completes the pleuro-peritoneal cavity. 



The heart consists at first of a single cylinder, which, in the 

 human embryo, probably is formed by the coalescence of two pri- 

 mary tubes. At first it has no distinct cavity, but soon the cells 

 of the mesoblast within the mass forming the heart become trans- 



