62 K. KISHINOUYE. 



are much smaller and more crowded than in other parts of the egg. 

 Thus the white disc is nothin«- more than a blastodermic thickenine:. 

 This blastodermic thickening is not produced by the addition of cells 

 from within, but by the rapid cell-division in a certain space of the 

 surface. The cells forming other parts of the egg, both superficial and 

 internal, are large, spherical, and all of about the same size. At this 

 stage all the cells contain yolk granules w^hich are smaller in the 

 vicinity of the nucleus. 



On August 8th (11 dnys after fertilization), the iDlastodermic 

 thickening had become larger and also deeper (fig. 22), but on the 

 next day it had increased onl}' in the horizontal plane. At about this 

 stage a thin transparent membrane, the vicarious chorion, is secreted 

 from the blastodermic cells. As the blastoderm-cells become pressed 

 together, and in consequence, polygonal grooves are formed around 

 each cell, the vicarious chorion is tucked in to fit these grooves and 

 presents polygonal markings with a double contour. 



On August loth (13 days after fertilization), an important change 

 had taken place. The cells forming the blastodermic thickening had 

 separated into two layers (tig. 24). The upper layer is composed of a 

 layer of cohunnar cells and represents the epiblast, the lower layer is 

 many cells thick and represents the mesoblast. Thus the blastodermic 

 thickening gives rise to the epiblast and mesoblast of the ventral plate. 

 The remaining germinal layer, the hypoblast is represented by the 

 large yolk-cells in the interior of the egg. Outside the ventral plate, 

 the epiblast rests directly on the hypoblast, the mesoblast being not 

 yet found in these parts. The epiblast cells throughout are smaller 

 than the hypoblast cells. Every cell of the egg had at this stage a 

 distinct nucleolus. Lute, on this day, the epiblast of the ventrum had 

 become composed of cells in many irregular rows, as a consequence 

 of rapid cell-proliferation (fig. 23). 



