DEVELOPMENT OF THE FROG. 549 



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. There are occasional variations 

 in the nature of the reproductive organs, and sometimes 

 the hermaphrodite stage through which the tadpoles pass 

 is to some extent retained. 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 even- 

 tually 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 gelatinous 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 left half. If one of these two cells be punc- 

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

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

 a certain stage Roux's half-embryo regenerated the missing 

 half, usually by re-vitalising the remains of the cell which 



