DEVELOPMENT OF REPRODUCTIVE SYSTEM 799 



tissue of the ovarian sac or enlarged space within the ovary (fig. 350J). 

 The ovary of the fully developed frog (and amphibian ovaries in 

 general) is saccular (Chap. 2). 



4. Development of the Reproductive Ducts 



Most vertebrate embryos, with the exception of those of teleost and certain 

 other fishes, develop two sets of ducts, one set of which later functions as 

 reproductive ducts. These ducts are the mesonephric. Wolffian or male 

 ducts and the Miillerian or female ducts. In the elasmobranch fishes, the 

 Mijllerian duct arises by a longitudinal division of the mesonephric duct (fig. 

 351 A). In the Amphibia, the Miillerian duct takes its origin independently. 

 Anteriorly it arises as a peritoneal invagination of the coelomic epithelium, 

 in the region of the cephalic end of the mesonephros. Posteriorly, this peri- 

 toneal invagination, as it grows caudally, appears to receive, in some urodeles, 

 contributions from the mesonephric duct (fig. 35 IB). In the Amniota 

 the MUllerian duct arises independently by a tubular invagination of the 

 coelomic epithelium at the anterior end of the mesonephric kidney (fig. 



Fig. 351 — (Continued) 

 (mesonephric) duct to join the cloaca (see fig. 351, C-2). The metanephric duct or 

 ureter arises as an evagination of the caudal end of the pronephric (mesonephric) duct 

 (see fig. 344A). C-2 is a drawing of the urogenital system of a 26 mm. pig embryo 

 viewed from the ventral aspect. Note extent of Miillerian duct growth caudalward. C-3 

 represents a generalized indifferent condition of the urogenital system of the mammal. 

 C-4 and C-5 are diagrams of later stages in the development of the female (C-4) and 

 the male (C-5). These conditions pertain particularly to human embryos. However, by 

 a division of the uterus simplex into a bicornate or duplex condition it may be applied 

 readily to other mammals. (C-6) Later arrangement of reproductive ducts and the 

 associated ovaries in the human female after the descent of the ovaries. Observe origin 

 of various ligaments. (In this connection see also fig. 3.) (C-7) Later development of 

 the reproductive duct-testis complex in the human male, during descent of the testis 

 into the scrotum. Observe origin of testicular ligaments. (See also fig. 3.) (D) Trans- 

 verse section through anterior end of the meSonephric kidney of 10 mm. pig embryo 

 presenting the Miillerian duct invagination of the coelomic epithelium covering the 

 mesonephros. E-N are diagrams showing the adult excretory and reproductive duct 

 relationships in various fishes. The urinary ducts are shown in black. (Redrawn and 

 modified from Goodrich, 1930, Studies on the Structure and Development of Vertebrates, 

 Macmillan and Co., Limited, London, after various authors.) 



It will be observed that in the male ganoid fish. Acipenser, the vasa efferentia extend 

 from a longitudinal testis duct through the anterior or genital part of the kidney to the 

 Wolffian (mesonephric) iduct. The Wolffian duct thus becomes a duct of Leydig as in 

 the frog. However, in teleosts, and in Protopterus and Polypterus, a separate genital 

 duct which opens into the caudal end of the mesonephric duct is evolved. Hence, the 

 Wolffian (mesonephric) duct in these forms functions as a urinary duct only. The 

 separation of the genital duct from the urinary duct, with the exception of the urogenital 

 sinus region at the posterior end, is a fundamental characteristic of most vertebrate male 

 reproductive systems, including many amphibia. In female fishes, fig. 351, I-N, as in 

 other vertebrates, the reproductive duct is always distinct from the urinary duct. The 

 exact homologies of the reproductive duct in forms such as Lepisosteus (Lepidosteus) 

 and teleosts (fig. 351. L-N) with the Miillerian duct in other verterbates is not clear. 



