120 DEVELOPMENT OF THE FROG. 



abundant in the lower than the upper half of the egg, and this 

 unequal distribution of food-yolk is the direct cause of the 

 unequal segmentation of the egg. The purpose of food-yolk is to 

 afford a supply of nutriment at the expense of which the earlier 

 developmental processes may be accomplished, until the young 

 animal is sufficiently advanced to obtain food for itself : and the 

 direct influence of this food-yolk will be to hinder rather than 

 to help these processes. 



We have seen above that the history of development of 

 an animal is to be regarded as a recapitulation of its pedigree : 

 and this explanation applies to the earliest stages equally with 

 the later ones. If it be true that an animal, such as a frog, 

 during its own development repeats its ancestral history, climbs 

 up its own genealogical tree, then the earliest phases of this 

 development must represent the earliest, i.e., the most remote 

 ancestors. On this view the imicellular condition of the e^g 

 is of great interest as indicating a similar unicellular condition 

 in some very remote ancestor ', i.e., as indicating that the higher 

 animals are descended from forms which, like the Protozoa 

 nowadays, remained throughout their lives single cells. 



E. The Germinal Layers. 



At the close of segmentation we have seen that the eg^ con- 

 sists of cells of two kinds : firstly, those of the upper half of the 



E w 



Fig. 24. Longitudinal vertical section of a frog embryo, showing com- 

 mencing invagination, x 28. ^ 



B, blastopore : E E, outer or epidermic layer of epiblast : E N, inner or 

 nervous layer of epiblast: SO, segmentation cavity : Y, yolk cells. 



