DEVELOPMENT OF THE FROG. 549 



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 pigmented, the lower has no 

 pigment and contains much yolk. This yolk-containing 

 hemisphere is the heavier, and consequently is ' always 

 the lower half of the egg, whatever position this may 

 be originally placed in. 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 inter- 

 spaces 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, the other will, according to Roux, form a one-sided 

 half-embryo. This fact, disputed by Hertwig, suggests that 

 the very first division of the frog's ovum is qualitative. At 

 a certain stage, Roux's half-embryo regenerates the missing 

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

 was punctured. The second cleavage is also vertical, and 

 at right angles to the first, dividing an anterior from a 

 posterior half. The third cleavage is equatorial, at right 

 angles to the first two, dividing the dorsal region from the 

 ventral. 



The segmentation is total but unequal, and results in the 

 formation of a ball of cells, those of the upper hemisphere 

 being smaller and more numerous than the yolk-laden cells 

 below. Within there is a small segmentation cavity. Since 



