* DEVELOPMENT OF THE FROG. 599 
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 Algz, 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 punc- 
Fic. 324.—Division of frog’s ovum.—After Ecker. 
The numbers indicate the number of cells or blastomeres. 
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 
was punctured. If the ovum be shaken about after punctur- 
ing, a readjustment of material is effected, and a half-sized 
embryo is formed (Morgan). 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 
