I.] THE BLASTODERM. 17 



somewhat raised ring, often spoken of as the germinal wall ; 

 underneath the area pellucida is a shallow space containing 

 a nearly clear fluid, to the presence of which the central 

 transparency seems to be due. The white spot in the middle 

 of the area pellucida appears to be the nucleus of Pander 

 shining through. 



Vertical sections of the blastoderm shew that it is formed 

 of two layers. The upper of these two layers is com- 

 posed, see Fig. 3, ep, of a single layer of cells, with their 

 long axes arranged vertically, adhering together so as to 

 form a distinct membrane, the edge of which rests upon 

 the white yolk. After staining with silver nitrate, this 

 membrane viewed from above shews a mosaic of uniform 

 polygonal cells. 



Each cell is composed of granular protoplasm filled with 

 highly refractive globules; in most of the cells an oval 

 nucleus may be distinguished, and is most probably present 

 in all. They are of a uniform size (about 9 /JL) over the 

 opaque and the pellucid areas. 



The under layer (Fig. 8, Z), is composed of cells which 

 vary considerably in diameter ; but even the smaller cells of 

 this layer are larger than the cells of the upper layer. 



They are spherical, and so filled with granules and highly 

 refractive globules, that a nucleus can rarely be seen in them: 

 in the larger cells these globules contain a highly refractive 

 body very similar to that present in the white yolk spheres, 

 from the smaller kinds of which indeed they are scarcely 

 distinguishable. 



The cells of this layer do not form a distinct membrane 

 like the cells of the upper layer, but lie as a somewhat 

 irregular network of cells between the upper layer and the 

 bed of white yolk on which the blastoderm rests. The 

 lowest are generally the largest; in addition we find a 

 few still larger cells generally separated by a small interval 

 from the remainder of the cells of the lower layer, and 

 resting directly upon the white yolk (Fig. 3, 6). These are 

 frequently spoken of as formative cells; they are however 

 similar in character and indeed connected by gradations 

 with the larger cells of the lower layer. Their mode of 

 formation during segmentation will be subsequently de- 

 scribed. ' 



E. 2 



