110 DEVELOPMENT OF THE FROG 



one of cell-division ; and the unequal rates at which the 

 different parts of the egg segment are to be regarded as due to 

 the retarding influence of the granules of food-yolk, which, 

 being themselves inert, must hinder the activity of the proto- 

 plasm in which they are imbedded. These granules of food -yolk 

 are more 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 pi-ocesses. 



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 develop- 

 ment must represent the earliest, i.e., the most remote ancestors. 

 On this view the unicellular condition of the egg is of great 

 interest as indicating a similar unicellular condition in some 



EN 

 Fig. 23 — Longi.udinal vertical seciion of a frog embryo, sliowing 



commencing invagination, x 28. 



B, blastopore ; EE, outer or epidermic layer of epiblast ; EN, inner 



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



