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 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 punc- 



FIG. 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 



