Development of the Fourth Day 237 



cloaca in the male, and almost completely dis- 

 appears in the adult. In the female, the right 

 Miillerian duct becomes rudimentary, while 

 the left duct becomes enlarged to form the 

 oviduct, its peritoneal opening persisting as 

 the funnel-hke opening of the adult oviduct. 



The ureter in the adult of both sexes is 

 formed as a narrow anteriorly directed diver- 

 ticulum from the posterior end of the Wolffian 

 duct. 



The reprodzictive organs. — The collection of 

 cells lying at the upper angle of the body- 

 cavity and somewhat below and to the out- 

 side of the aorta (Fig. 73) has already been 

 spoken of as the intermediate cell-mass ; and 

 it has been mentioned that by the fourth 

 day this intermediate cell-mass projects, as 

 a rounded ridge, into the upper part of 

 the body-cavity, and may be now called the 

 genital ridge, or Wolffian ridge. The term 

 " genital ridge " is applied, by some workers, 

 to the whole ridge that fills the upper angle of 

 the body-cavity on each side : other authors use 

 the term " Wolffian ridge " for the entire ridge, 

 and restrict the term genital ridge to the inner 

 part of the large ridge, next to the splanch- 

 nopleure, where the reproductive organs are 



