670 PHYLUM CHORDATA : CLASS AMPHIBIA 



near which the oviducts open. The movement of the ova 

 is mainly due to the action of peritoneal cihated 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- 

 v/alled 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 

 eventually degenerates. It does not seem to have any- 

 thing 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. 



The spheres of jelly preserve the eggs and embryos from 

 friction, prevent their being eaten by most birds, appear to 

 be distasteful to Gammarids, and often enclose in their 

 interspaces groups of green Algae, which help in aeration. 

 The spheres may also be of use in relation to the absorption 

 and radiation of heat. 



Fertilisation occurs immediately after the eggs are laid. 

 The spermatozoa, which exhibit the usual features of male 

 elements, work their way through the gelatinous envelopes, 

 and one fertilises each ovum. 



The first cleavage is vertical, and divides the ovum into 

 a right and a left half. If one of these two cells be punctured, 

 and the ovum be kept still, the other half will, according 

 to Roux, form a one-sided half-embryo. At a certain 



