DEVELOPMENT OF THE FROG, 



579 



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 



1 2 4 8 



FIG. 283. Division of frog's ovum. After Ecker. 

 The numbers indicate the number of cells or blastomeres. 



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- 



