I.] THE BLASTODERM. 



rests. The lowest are generally the 

 largest. The layer is thicker at the peri- 

 phery than at the centre : and rests on 

 a bed of white yolk, from which it is in 

 parts separated by a more or less de- 

 veloped cavity, containing probably fluid 

 yolk matter about to be absorbed. In 

 the bed of white yolk nuclei are present, 

 which are destined to become the nuclei 

 of cells about to join the lower layer of 

 the blastoderm. These nuclei are gene- 

 rally more numerous in the neighbour- 

 hood of the thickened periphery of the 

 blastoderm than elsewhere. Amongst 

 the lower layer cells are to be found 



FIG. 3. 

 SECTION OF A BLASTODERM OF A FOWL'S EGG 



AT THE COMMENCEMENT OF INCUBATION. 



The thiii but complete upper layer ep com- 

 posed 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 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 under- 

 stand 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. 



