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 &. esculenta the vas deferens is.dilated for 
some distance after leaving the kidney; in &. temporaria 
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. 
