46o AMPHIBIA. 



on its outer side near the cloaca a dilated glandular mass or 

 " seminal vesicle." In the males rudiments of the Miillerian 

 ducts are sometimes seen. 



The paired ovaries when mature are large plaited organs, 

 bearing numerous follicles or sacs containing the pigmented 

 ova. The ripe ova are liberated into the body-cavity, and 

 moved anteriorly towards the heart near which the oviducts 

 open. These 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 gelatinous stuff; the terminal uterine parts 

 open on the dorsal wall of the cloaca. In the females the 

 Wolffian ducts act solely as ureters. There are occasional 

 variations in the nature of the reproductive organs, and 

 sometimes the hermaphrodite stage through which the tad- 

 poles pass, is to some extent retained, causing partial herma- 

 phroditism. Attached to the anterior end of the reproductive 

 organs, are yellow lobed " fatty bodies." They are always 

 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 even- 

 tually degenerates. It does not seem to have anything to 

 do with the fatty bodies. 



Development of the Frog. — The ripe ovum contains a con- 

 siderable quantity of yolk which has sunk towards one pole. 

 The upper hemisphere contains much black pigment, the 

 lower is not pigmented. If the ovum be artificially fixed so 

 that the yolk-laden part is uppermost, the heavier material 

 will again sink downwards. Round the egg there is a sphere 

 of clear gelatinous stuff, which swells in the water. Before 

 the liberation of the eggs, nuclear changes occur which seem 

 to correspond to the formation of polar bodies. 



Fertilisation takes place, as we have already mentioned, 

 immediately after the liberation of the eggs from the mother 

 frog. The spermatozoa, which exhibit the usual features of 

 male elements, work their way through the gelatinous en- 

 velopes of the ova, and one fertilises each ovum. The 

 segmentation which follows is total, but unequal, resulting in 

 a ball of cells, those of the upper hemisphere being smaller 



