510 STANLEY C. BALL 



farther advanced. These three figures are from preparations 

 stained with iron-haematoxylin and eosin after fixation in 

 Flenuning's solution. 



To conclude the description of the stage seen in figure 23, the 

 pair of cells mentioned above (fig. 22) as being reserved during 

 the early period of ectoderm differentiation for the development 

 of the internal pharyngeal epithelium, are conspicuous at the 

 anterior end of the embryo. Their cytoplasm remains con- 

 sistently lighter and clearer than that of the adjacent interior 

 cells. Their resemblance is rather on the side of the ectoderm. 



Text fig. 15 Surface view of an embryo forcibly freed from its capsule shortly 

 after gastrulation. Anterior end at the right. X 530. 



Figure 25, drawn from a fresh capsule, shows the appearance 

 of a living embryo at a stage slightly later than that in section 

 (fig. 23). Through the transparent ectoderm can be seen the 

 prominent yolk-filled cells of the primary entoderm. The 

 highly refractive bodies so conspicuous in the ectoderm cells 

 are regarded as globules of the same nature as those constituting 

 the free yolk still remaining in the capsule. The larger opaque 

 masses in the posterior ectoderm cells are the homologues of 

 those noted above in figure 22, i.e., masses of yolk undergoing 

 digestion. 



Between the capsular shell and the enclosed embryos and 

 yolk granules is a clear zone filled with a colorless fluid. Prob- 



