DEVELOPMENT. 771 



a layer of columnar nucleated cells, and the lower layer consists of larger 

 masses indistinctly nucleated, still granular and rounded, irregularly 

 disposed. In the segmentation cavity are the supplementary segmenta- 

 tion masses or formative cells. 



When the egg is incubated, rapid changes take place in the blasto- 

 derm, resulting in the formation first of all of two, then of the three 

 layers, which have been already mentioned in the first chapter. The 

 superficial, or epiblast, does not at first enter into these changes, but 



Fig. 462. Impregnated egg, with commencement of formation of embryo; showing the area, 

 germinativa or embryonic spot, the area pellucida, and the primitive groove or trace. 

 (Dalton.) 



continues to be a layer of nucleated columnar cells. But in the lower 

 layer of larger rounded cells certain of the cells become flattened hori- 

 zontally, their granules disappear, and the nuclei become distinct. A 

 membrane of flattened nucleated cells is then formed, first of all toward 

 the centre of the area, afterward peripherally also : this is the hypoblast. 

 Between the two layers some cells, not belonging to either layer, remain. 

 These cells are almost entirely at the back part of the area. The for- 

 mation of the intermediate layer of mesoblast is more complicated, 

 and will now be described. 



At this period it is necessary to return to the surface view of the 

 blastoderm. Before incubation it is seen to consist of a more or less 

 circular transparent area, the area pellucida, surrounded by an opaque 

 rim, which is called the area opaca. The area opaca rests upon the 

 white yolk : beneath the area pellucida is a cavity containing fluid. In 

 the centre of the area pellucida is a white shining spot, or nucleus of 

 Pander, shining through. This is the upper dilated extremity of the 

 flask-shaped accumulation of white yolk upon which the blastoderm 

 rests. 



The yellow yolk consists of spheres %5/j. to 100//. in diameter, filled with 

 highly refractive granules of an albuminous nature, and the white yelk 

 being distinguished from the yellow not only by its lighter color, but 

 also because its vesicles are smaller than those of the yellow. Each con- 



