834 GENERATION. 



rior extremity of the body of the embryon 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 developed internally a septum, 

 which separates the rectum from the vagina, the urethra of the female open- 

 ing above. In the male this septum is developed between the rectum and the 

 urethra, the generative and the urinary 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 cor- 

 pora 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. When there is a want of 

 union in the cavernous bodies in the male, there is the malformation known 

 as hypospadias. 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 farther illustrated by the 

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

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

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

 and pass through the external abdominal ring. 



DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



The blood and the blood-vessels are developed very early in the life of the 

 ovum and make their appearance nearly as soon' as the primitive trace. 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, ves- 

 sels are formed as prolongations of pre-existing tubes. Soon after the epi- 

 blast and the hypoblast have become separated from each other and the 

 mesoblast has been formed at the thickened portion of the ovum, which is 

 destined to be developed into the embryon, certain of the blastodermic cells 

 undergo a transformation into blood-corpuscles. These are larger than the 

 blood-corpuscles of the adult and generally are nucleated. At about the 

 same time it may be before or after the appearance of the corpuscles, for 

 this point is undetermined certain of the blastodermic cells fuse with each 

 other and arrange themselves so as to form vessels. Leucocytes probably are 

 developed in the same way as the red corpuscles. The vessels thus formed 

 constitute the area vasculosa, which is the beginning of what is known as 

 the first circulation. 



According to His and Waldeyer, the cells of the mesoblast do not take 

 part in the formation of the blood and blood-vessels, as indicated above, but 

 cells penetrate at the edges, between the epiblast and the hypoblast, and these, 

 which are called parablastic cells, are developed into blood-vessels and blood- 

 corpuscles. The connective tissue is also supposed to be developed from 

 parablastic cells. According to this view which, however, is not generally 



