DEVELOPMENT 



457 



body) also contribute to the excretory duct system. In the male, they per- 

 sist in part (10 to 15) to form the ductuli efferentes; several may become 

 vestigial forming thus the ductuli aberrentes, the paradidymis and the 

 appendix epididymis. In the female, these ducts early disappear for the 



Inguinal ligament 



Mesonephric due 

 MuUerian duct 



--Apex of bladder 

 Bladder 



Opening of ureter 



( Opening of mesoneph- 



\ ric duct 



( Opening of MuUerian 



\ duct 

 Rectum 



Urogenital sinus 

 Cloaca 



Genital tubercle 

 Genital ridge 

 Opening of cloaca 



FIG. 420. DIAGRAMS ILLUSTRATING THE METAMORPHOSES OF THE INDIFFERENT 

 (A) UROGENITAL SYSTEM INTO THE MALE (B), AND FEMALE (C) SYSTEMS. (From 

 Polak, after Hertwig.) 



most part a variable number persisting as vestigial structures: the epo- 

 ophoron and the parob'phoron. 



The seminiferous tubules (including the recti and rete tubuli) of the 

 testis arise as solid cords of cells continuous with the peritoneal epithelial 

 covering of the gonad, and apparently as derivatives of this so-called 'ger- 

 minal epithelium.' These cords subsequently acquire a lumen, and connect 

 with the efferent tubules. In the ovary likewise such cell cords (sex-cords) 

 appear, continuous with the peritoneal (germinal) epithelium; from these 

 develop the ovarian follicles with their ova, as will be described below. 

 The relation of the extra-regional 'primordial germ-cells' to the germinal 

 epithelium and to sex-gland derivatives, discussed in the next section, 



