DEVELOPMENT OF THE CIRCULATORY SYSTEM 839 



adult that they have been supposed to be peculiarly important in intra- 

 uterine life, although nothing definite is known on this point (see Plate 

 XVI, Fig. 3). The kidneys are relatively very large in the foetus. 

 Their proportion to the weight of the body, in the foetus, is i to 80, and 

 in the adult, I to 240. The ureters are developed as tubular processes 

 from the kidneys, which finally extend to open into the bladder. The 

 development of the genito-urinary apparatus can be readily understood, 

 after the description just given, by a study of Fig. 312. 



Development of the External Organs of Generation. The external 

 organs of generation begin to be developed at about the fifth week. At 

 the inferior extremity of the body of the embryo, a small ovoid eminence 

 appears in the median line, at the lower portion of which there is a 

 longitudinal slit which forms the common opening of the anus and the 

 genital and urinary passages. This is the cloaca. There is soon devel- 

 oped internally a septum, which separates the rectum from the vagina, 

 the urethra of the female opening above. In the male this septum is 

 developed between the rectum and the urethra, the generative and the 

 uninary passages opening together. From this median prominence two 

 lateral rounded bodies make their appearance. These are developed, 

 with the median elevation, into the glans penis and corpora cavernosa 

 of the male or into the clitoris and the labia minora of the female. In 

 the male these two lateral prominences unite in the median line and 

 enclose the spongy portion of the urethra. In the female there is no 

 union in the median line, and an opening remains between the two 

 labia minora. The scrotum in the male is analogous to the labia majora 

 of the female ; the distinction being that the two sides of the scrotum 

 unite in the median line, while the labia majora remain permanently 

 separated. This analogy is further illustrated by the anatomy of 

 inguinal hernia, in which the intestine descends into the labium in the 

 female or into the scrotum in the male. It sometimes occurs, also, 

 that the ovaries descend, much as the testicles pass down in the male, 

 and pass through the external abdominal ring. 



DEVELOPMENT OF THE CIRCULATORY SYSTEM 



The blood and the bloodvessels are developed very early in the life 

 of the ovum and make their appearance nearly as soon as the primitive 

 streak. The mode of development of the first vessels differs from that 

 of vessels formed later, as they appear de novo in the blastodermic 

 layers, while afterward, vessels are formed as prolongations of preexist- 

 ing tubes. Soon after the epiblast and the hypoblast have become 

 separated from each other, and the mesoblast has been formed at the 



