10 THE hen's egg. [chap. 



peculiar large spherical bodies, which superficially re- 

 semble the larger cells around them, and have been 

 called formative cells. Their real nature is still very 

 doubtful, and though some are no doubt true cells, 

 others are perhaps only nutritive masses of yolk. 



The opacity of the peripheral part of the blastoderm 

 is in a large measure due to the collection of the lower 

 layer cells in this region, and the thickening, so caused, 

 appears to be more pronounced for a small arc which 

 subsequently constitutes the hinder border of the area 

 pellucida. 



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. 



To recapitulate : In the normal unincubated hen's 

 esfff we recognize the blastoderm, consisting of a com- 

 plete 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, and a peripheral thickening of the 

 lower layer causes the appearance in the blastodermic 

 disc of an area opaca and an area pellucida. The great 

 mass of the entire yolk consists of the so-called 

 yellow yolk composed of granular spheres. The 

 white yolk is composed of smaller spheres of pecu- 

 liar structure, and exists, in small part, as a thin 

 coating around, and as thin concentric laminae in 

 the substance of the yellow yolk, but chiefly in the 



