and very convoluted; it extends through the entire length 

 of the body cavity. The eggs enter the ostium and traverse 

 this duct, where they acquire their jelly coating in ovipa- 

 rous types. In viviparous types the eggs develop in this tube. 

 Several species of Salamandra and Spelerpes fuscus are vivip- 

 arous, as are the apodans Typhlo7iectes compressicauda and 

 Dermophis thomensis. The tubes can be divided into an ostium, 

 an infundibulum (with a thin wall, wide lumen, and no 

 glands), a tube (with glands in those forms which lay eggs, 

 or no glands but mucous cells in the viviparous types), a 

 uterus (with a much folded epithelium forming a reticular 

 pattern and a wide lumen), and a vagina. The last is a short 

 section between uterus and cloaca. The MUllerian ducts 

 open into the cloaca on a papilla lateral to the opening of 

 the nephric ducts. These ducts arise as in the higher forms. 



Genera/ observations In the development of the amphi- 

 bian gonad, the genital ridge forms on the ventromedial or 

 medial aspect of the mesonephric kidney bulge. This ridge 

 extends much of the length of the mesonephros and may be 

 considered as having three areas: anterior progonalis, mid- 

 dle gonalis, and posterior epigonalis (Figure 10-40). The 

 definitive gonad develops from the middle area in all tetra- 

 pods. The fat bodies of urodele and apodan represent a part 

 of the entire band, whereas in the anuran, only the progonalis 

 gives rise to the fat body. 



Bidder's organ develops in all males and some females of 

 the toad family, Bufonidae. This organ develops from the 

 gonalis section just anterior to the definitive gonad. The 

 function of the organ is not known but is assumed to be en- 

 docrine since this structure undergoes an annual cycle of 

 size change, decreasing in size prior to the reproductive 

 period. This change in size is more marked in the female 

 than in the male, but since females of only a few species 

 have the organ while all of the males have it, this difference 

 in size change is not as meaningful as might be assumed at 

 first glance. Bidder's organ is known also to have the capa- 

 bility of developing into an ovary when either sex is cas- 

 trated. 



In the development of the gonad, germ cells first appear 

 as a band just dorsal to the enteron (gut cavity), ventral to 

 the aorta, and between the two lateral mesoderm plates. This 

 band of yolk-laden germ cells lies at the dorsal root of the 

 gut mesentery and can be observed in the serial sections of 

 the 10-mm frog (Figure 10-16 C). 



The germ ridge separates into two strands which move 

 laterally and form slight folds on either side of the median 

 line. These folds constrict basally so that a suspensory liga- 

 ment is formed. As seen in cross section, each gonad consists 

 of one or two large, yolk-laden germ cells with a few small 

 heavily staining peritoneal cells around them. The whole is 

 enclosed by the germinal epithelium. 



FEMALE 



MALE 



APODA URODELA BUFONIDAE RANIDAE 



fat body 



RANIDAE BUFONIDAE URODELA 



fat body 



APODA 



^mesentery 



Figure 10-40. Relationships of gonad, fat body, and Bidder's organ derived from gonadal ridge in amphib- 

 ians. Cross sections show interrelationships to mesentery of gonadal ridges. For orientation, the midline of the 

 page corresponds to the midline of the animal and the margins are lateral. (After van den Broeck, 1933) 



326 • THE UROGENITAL SYSTEM 



