BLOOD-VASCULAR SYSTEM. 693 



urinary portion. The septum, dividing the urogenitary from 

 the alimentary portion of the cloaca, forms, externally, the peri- 

 nseum. 



At the aperture of the cloaca an eminence arises which de- 

 velops 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 yelk sack ; and the second, or later cir- 

 culation, 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 fore part 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 backward toward the 

 tail end of the embryo. This groove is limited behind by the 

 somato-pleural layer of the mesoblast, and posteriorly to this is 

 a cavity formed by the cleavage of the mesoblast, called the 

 pleuroperitoueal cavity. In the earlier stages of development, 

 the posterior wall of this small cavity is formed by the splanch- 

 nopleural layer of the mesoblast. The heart arises at the point 

 at which the splanchnopleure tends to travel forward to meet 

 the uncleft mesoblast, and thus completes the pleuroperitoneal 

 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- 



