50 



THE FIRST DAY. 



[CHAP. 



FIG. 12. 



i 



SECTION OF A BLASTODERM OF A FOWL'S EGG 

 AT THE COMMENCEMENT OF INCUBATION. 



The thin but complete upper layer ep 

 composed of columnar cells rests on the in- 

 complete lower layer I, composed of larger 

 and more granular cells. 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 sur- 

 face 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 the left ep is continued farther than 

 I, so that at the extreme edge it rests directly 

 on the white yolk. 



It will be remembered that the 

 blastoderm in the unincubated egg is 

 composed of two layers, an upper (Fig. 

 12, ep) and an under layer; that the 

 upper is a coherent membrane of colum- 

 nar nucleated cells, but that the lower 

 one (Fig. 12, 1) is formed of an irregular 

 network of larger cells in which the 

 nuclei are with difficulty visible; and 

 that in addition to this there are certain 

 still larger cells, called 'formative cells' 

 (Fig. 12,6), lying at the bottom of the 

 segmentation-cavity. 



Under the influence of incubation 

 changes take place very rapidly, which 



