50 



which appear at the sides, and also in the middle a few sections back 

 of this point, there are more or less globular cells having their 

 nuclei near their centers. They are undergoing rapid division, as 

 is shown by the fact that a large number are seen in various stages 

 of mitosis. These cells, including about the middle one-third of the 

 germ band, are seen to be sunk below the level of the columnar cells 

 on either side, forming a broad shallow depression. This depression, 

 which has been termed by some embryologists the iiiiddle plate, rep- 

 resents an invagination from which will arise the mesoderm. At 

 this stage the middle plate occurs only near the anterior end. 



In a stage a day later (three days old) the small area on the 

 dorsal side that still remained uncovered by the blastoderm has become 

 overgrown by the layer of cells, so that we now have the blastoderm 

 and the germ band completely enclosing the yolk (Fig. 12). Sections 

 through the egg at this stage show us that the cells of the blastoderm 

 are not all alike. In the first place we have in the anterior half of 

 the ventral surface, the germ band, the surface cells of which are 

 columnar and stain deeply with hsematoxylin (Fig. 12). Just pos- 

 terior to the germ band comes a layer of large polygonal cells con- 

 taining only a small amount of protoplasm in comparison with the 

 large amount of yolk material within them. The protoplasm, which 

 is rather dense, but does not stain so^ deeply as in the cells of the 

 germ layer, is all together, while the yolk granules fill the rest of 

 the cell. This layer extends almost to the posterior end, but just 

 before that end is reached a few enormously enlarged cells occur 

 which closely resemble those just described, but are conspicuous be- 

 cause of their great size. They appear to be multinucleate, although 

 their nuclei do not show up very well, and to enter into close relation- 

 ship with the posterior end of the inner protoplasmic layer. They con- 

 tain a number of vacuoles, and, like the cells just described, they 

 contain a very large amount of yolk material, and their rather dense 

 protoplasm stains more lightly than that in the cells of the germ 

 band. At the posterior end, and extending forward from these al- 

 most half the distance on the dorsal side, occurs a layer of rather 

 large polygonal cells which contain only a few yolk granules. The 

 protoplasm of these cells is less dense than that of the cells just de- 

 scribed, but it takes a deeper stain, and hence these cells are very easily 

 distinguished from the others. About half-way between the anterior 

 and posterior ends, these cells give way to cells of the same kind 

 as those which occur just posterior to the germ band on the ventral 

 surfaces. These cells extend forward on the dorsal surface to meet 

 the germ band at the anterior end of the blastoderm. 



