112 DEVELOPMENT OF THE FROG 



different parts of the egg segment, are to be regarded ai 

 due to the retarding influence of the granules of food-yolk 

 which, being themselves inert, must hinder the activity o! 

 the protoplasm in which they are imbedded. These granule: 

 of food-yolk are more abundant in the lower than the uppei 

 half of the egg, and this unequal distribution of food-yell 

 is the direct cause of the unequal segmentation of the egg 

 The purpose of food-yolk is to afford a supply of nutrimen 

 at the expense of which the earlier developmental processe 



SC 



FIG. 23. Longitudinal vertical section of a frog embryo, showing 

 commencing hypoblast at B. x 28. 



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

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



may be accomplished, until the young animal is sufficient! 

 advanced to obtain food for itself; and the direct influenc 

 of this food-yolk will be to hinder rather than to help thes 

 processes. 



E. The Germinal Layers. 



At the close of segmentation we have seen that the eg, 

 consists of cells of two kinds ; firstly, those of the upper h-xi 

 of the egg, which are smaller, pigmented, more regula'-l 

 arranged, and comparatively free from food-yolk second!) 



