THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



427 



changes that occur in the female. A careful study of the diagrams will assist 

 the student materially in understanding the processes of development which 

 have been described in the preceding paragraphs. 



Below is a table that is meant to set forth briefly the various structures 

 which belong to the internal genital organs in the two sexes, and which are 

 derived from the structures in the "indifferent" stage. The words in italics' 

 are the names of structures that persist in a rudimentary form. 



THE EXTERNAL GENITAL ORGANS. 



In addition to the internal organs of generation, to which the description has 

 thus far been confined, certain other structures appear on the outside of the 

 body to form the external genitalia. In the case of these also there is an "indif- 

 ferent" stage from which the courses of development diverge in the two sexes. 



During the sixth week a depression appearing on the ventral surface of the 

 caudal end of the body indicates the position of the cloacal membrane (p. 403) . 

 This becomes surrounded by a slight elevation, produced by the thickening 

 of the mesoderm which is known as the genital ridge (Fig. 381). The cephalic 



