204 HUBERT DANA GOODALE 



endoderm cells, en, are larger and usually arranged in a single 

 layer. In the mid-dorsal region, both endoderm and mesoderm 

 graduate insensibly into a mass of indifferent cells, arranged in 

 three or four layers and about the size of the mesoderm cells. 

 Its middle part is the forerunner of the notochord, n, while the 

 cells at the side may become either mesoderm, endoderm, or chorda. 

 This arrangement exists throughout the posterior half of the em- 

 bryo, xlnteriorly, the cells are still undifferentiated. 



The section figured is the 36th from the blastopore in the series of 

 106. The mesoderm can be made out up to the 60th section. On the 

 left side of the drawing, the mesoderm is not so distinctly differentiated. 

 In the actual sections the cell walls of the mesoderm cells are as a rule 

 better developed and hence more sharply stained than those of the endo- 

 derm. This distinction is naturally lost in the drawings. 



The differentiation of the yolk cell§ of the archenteron into 

 mesoderm and endoderm, which, it must be admitted, is a little 

 vague in fig. 37, is fully verified in the next, fig. 38, which rep- 

 resents the 30th section from the posterior end of the egg. The 

 mesoderm can be followed to the 60th section. Its cells, m, are 

 rounded, much smaller than the endoderm cells and usually form 

 a double row. The latter's cells, en, are somewhat elongated 

 and attached end to end. As shown in the figure, this layer as 

 a whole is easily pulled away from the mesoderm in sectioning. 

 This frequently happens and always in the same manner, indicat- 

 ing differences in cohesion of the various layers. The cells of 

 the mid-dorsal region, n, have become columnar and clearly 

 form the anlage of the notochord. At either side of the notochord 

 the cells are irregular and all three kinds, viz. : notochord, meso- 

 derm and endoderm are fused together. At the sides below the 

 level of the floor of the archenteron, the mesoderm cells can be 

 followed for a short distance. On the right side in one place, 

 they are not much different from the neighboring yolk cells from 

 which they are probably differentiating. There is a large archen- 

 teron, a, which is even larger in the anterior regions (cf. fig. 39, 

 which, while not drawn from the same egg, is from an egg at the 

 same point of development. This section is the 63rd in 106). 



