8 THE hen's egg, [chap. 



beginning of the area pellucida. In the part corre- 

 sponding to the area opaca the blastoderm rests imme- 

 diately on the white yolk ; underneath the area pellu- 

 cida is a shallow space containing a nearly clear fluid, 

 to the presence of which the central transparency seems 

 to be due. The white spot in the middle of the area 

 pellucida appears to be the nucleus of Pander shining 

 through. 



Vertical sections of the blastoderm shew that it is 

 formed of two layers. The upper of these two layers 

 is composed, see Fig. 3, ep, of a single row of cells, 

 with their long axes arranged vertically, adhering 

 together so as to form a distinct membrane, the edge of 

 which rests upon the white yolk. After staining Avith 

 silver nitrate, this membrane viewed from above shews 

 a mosaic of uniform polygonal cells. 



Each cell is composed of granular protoplasm filled 

 with highly refractive globules ; and in each an oval nu- 

 cleus may be distinguished. They are of a nearly uniform 

 size (about 9 fi) over the opaque and the pellucid areas. 



The under layer (Fig. 3, t), is composed of cells 

 which vary considerably in diameter; but even the 

 smaller cells of this layer are larger than the cells of the 

 upper layer. They are spherical, and so filled with 

 granules and highly refractive globules, that a nucleus 

 can rarely be seen in them : in the larger cells these 

 globules are identical with the smaller white yolk 

 spheres. 



The cells of this layer do not form a distinct mem- 

 brane like the cells of the upper layer, but lie as a 

 somewhat irregular network of cells between the upper 

 layer and the bed of white yolk on which the blastoderm 



