is 



THE HEN S EGG. 



[chap. 



Tig. 3. 



Section op a Blastoderm op a Fowl's Egg at the 

 commencement op incubation. 



The thin but complete upper layer ep composed of 

 columnar cells rests on the iucomplete lower layer /, 

 composed of larger and more granular bodies. The lower 

 ,-.-,„ layer is thicker in some places than in others, and is 



lO r^x especially thick at the periphery. The line below the 



under layer marks the upper surface of the white yolk. 

 The larger so-called formative cells are seen at 6, lying 

 on the white yolk. The figure dues not take iu quite 

 the whole breadth of the blastoderm ; but the reader must 

 understand that both to the right hand and to the left ep 

 is continued faither than /, so that at the extreme edge 

 it rests directly on the white j olk. 



Over nearly the whole of the blastoderm 

 the upper layer rests on the under layer. 

 At the circumference however the upper 

 layer stretches for a short distance beyond 

 the under layer, and here consequently rests 

 directly on the white yolk, and forms that 

 part of the blastoderm known as the area 

 opaca. 

 ,^:^ja 10. To recapitulate: — In the normal 



\i^ unincubated hen's egg we recognize the 



blastoderm, consisting of a complete upper 

 layer of smaller nucleated granular cells and 

 a more or less incomplete under layer of 

 larger cells, filled with larger granules ; in 

 these lower cells nuclei are rarely visible. 

 The thin flat disc so formed rests, at the 

 uppermost part of the entire yolk, on a bed 

 of white yolk so disposed as to give rise to 

 the appearance in the blastodermic disc it- 

 self of an area opaca and an area pellucida. 

 The great mass of the entire yolk consists of 

 the so-called yellow yolk composed of gra- 

 nular spheres. The white yolk is composed 

 of smaller spheres of peculiar structure, and 

 exists, in small part, as a thin coating around, 

 and as thin concentric laminae in the sub- 

 '^ ^ stance of the yellow yolk, but chiefly in the 



form of a flask-shaped mass in the interior 

 of the yolk, the upper somewhat expanded top of the neck 



