18 



THE HEN'6 EGG. 



[CHAP. 



FIG. 3. 



SECTION OF A BLASTODEBM OF A FOWL'S EGG AT THE 



<foMMENCEMENT OF INCUBATION. 



The thin but complete upper layer ep composed of 

 columnar cells rests on the incomplete lower layer /, 

 composed of larger and more granular bodies. The lower 

 layer is thicker in some places than in others, and is 

 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 does not take in 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 farther than /, so that at the extreme edge 

 it rests directly on the white }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* 



10. To recapitulate : In the normal 

 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 



