226 THE MESODERMAL DERIVATIVES 



mesonephric duct. There is therefore no separate vas deferens, this 

 mesonephric or Wolffian duct acting in the normal capacity of a vas 

 deferens. In the male frog, then, it is a true urogenital duct from the 

 beginning. 



As the mesonephric (Wolffian duct or vas deferens) duct reaches 

 the cloaca it enlarges and becomes somewhat glandular, and is known 

 as the seminal vesicle. It is within this vesicle that the sperm masses 

 are retained during amplexus and from which they are ejected into 

 the water during oviposition. 



The Miillerian duct, which is the male homologue of the oviduct, 

 develops late in embryogenesis from a longitudinal ridge of cells 

 which project into the coelomic cavity just ventral and lateral to the 

 pronephric region and the Wolffian duct. This ridge sometimes de- 

 velops a tube and extends from the cloaca to a point near the junc- 

 tion of the lungs, anterior liver lobes, and heart. It is covered with 

 and suspended by a thin layer of peritoneal epithelium. Originally 

 this Miillerian duct was described as originating from a longitudinal 

 splitting of the segmental duct, but this has been doubted recently. 

 The duct often degenerates in the male adult but there may be vestiges 

 in higher vertebrates such as the appendix, testis, and prostatic utricle. 

 Its homology to the female oviduct can be demonstrated by treating 

 the male with estrogens which cause it to hypertrophy, and to have 

 the appearance of an oviduct. 



The Female. The mesonephric or Wolffian duct of the female func- 

 tions exclusively as a ureter. The oviducts arise in a manner similar 

 to the Miillerian ducts of the male but they acquire a lumen which is 

 surrounded by ciliated and glandular epithelium and muscular walls. 

 It is suspended to the dorsal body wall by a double fold of peritoneum. 

 The anterior end of each oviduct consists of a persistent group of 

 nephrostomes which become the fimbriated and highly elastic infundib- 

 ulum or ostium tuba. The oviduct shows slight differential gland 

 development and convolutions from the anterior to the posterior, 

 where it becomes the very distensible uterus. The two uteri open sepa- 

 rately into the cloaca. 



The body cavity of the female develops an abundant supply of 

 cilia, in response to the appearance of an ovarian hormone. These 

 cilia may therefore be regarded as secondary sexual characters of 

 the female, appearing shortly after the time of metamorphosis. 



The Gonads. The paired gonads arise as a single median sex cell 



