598 AMPHIBIA. 



through the anterior part of the kidney into the Wolffian 

 duct, which functions both as a ureter and as a vas deferens. 

 In the male of R. esculenta the vas deferens is dilated for 

 some distance after leaving the kidney ; in R. temporaries 

 it bears on the outer side near the cloaca a dilated glandular 

 mass or "seminal vesicle." In the males, rudiments of the 

 Miillerian -ducts are sometimes seen. In the male toad a 

 small rudimentary ovary, known as Bidder's organ, occurs 

 at the anterior end of the testis. 



The paired ovaries when mature are large plaited organs, 

 bearing numerous follicles or sacs containing the pigmented 

 ova. The spawn laid by a single frog may consist of several 

 thousand eggs. The ripe ova are liberated into the body 

 cavity, and moved anteriorly towards the heart, near which 

 the oviducts open. The movement of the ova is mainly 

 due to the action of peritoneal ciliated cells, which converge 

 towards the mouths of the oviducts, but partly to muscular 

 contraction, including the beating of the heart. The 

 oviducts are long convoluted tubes, anteriorly thin-walled 

 and straight, then glandular and coiled, terminally thin- 

 walled and dilated. In the median part the ova are 

 surrounded with jelly ; the terminal uterine parts open on 

 the dorsal wall of the cloaca. In the females the Wolffian 

 ducts act solely as ureters. Attached to the anterior end of 

 the reproductive organs are yellow, lobed, " fatty bodies," 

 largest in the males. It has been suggested that they 

 contain stores of reserve material, which is absorbed at 

 certain seasons. They seem to be fatty degenerations of 

 the anterior part of the genital ridges. The head kidney or 

 pronephros persists for some time in the embryo, but event- 

 ually degenerates. It does not seem to have anything to 

 do with the fatty bodies. 



Development of the frog. The ripe ovum exhibits 

 " polar differentiation " ; its upper portion is deeply pig- 

 mented, the lower has no pigment and contains much yolk. 

 This yolk-containing hemisphere is the heavier, and conse- 

 quently is always the lower half of the egg, however this may 

 be turned about. Round the ovum there is a delicate 

 vitelline membrane, and this is again surrounded by a gela- 

 tinous investment which swells up in water. The formation 

 of polar bodies takes place before the liberation of the eggs. 



