36 EARLY EMBRYOLOGY OF THE CHICK 



The cells of the entoderm are neither as closely packed nor as 

 clearly defined as are the ectoderm cells. Nevertheless, in 

 contrast to the condition of the mesoderm at this stage, the 

 entoderm cells form a definite, unbroken layer. 



The Growth of the Entoderm and the Establishment of 

 the Primitive Gut. Sections of embryos of this stage show 

 how the entoderm has spread out and become organized into a 

 coherent layer of cells merging peripherally with the inner mar- 

 gin of the germ wall and overlapping it to a certain extent 

 (Fig. 13, C, E, F). The cavity between the yolk and the ento- 

 derm which has been called the gastroccele is now termed the 

 primitive gut. The yolk floor of the primitive gut does not 

 show in sections prepared by the usual methods. The reasons 

 for this are to be found in the relations of the embryo to the 

 yolk before it is removed for sectioning. In the entire central 

 region of the blastoderm the yolk is separated from the ento- 

 derm by the cavity of the primitive gut. When the embryo is 

 removed from the yolk sphere the yolk floor of the primitive 

 gut, not being adherent to the blastoderm, is left behind. In 

 contrast the peripheral part of the blastoderm lies closely ap- 

 plied to the yolk. Some yolk adheres to this part of the blasto- 

 derm when it is removed. This adherent yolk is shown in the 

 section diagrams of Figure 13. Its presence clearly indicates 

 why this region (area opaca) appears less translucent in surface 

 views of entire embryos. 



In embryos of 18 hours the primitive gut is a cavity with 

 a flat roof of entoderm and a floor of yolk. Peripherally it is 

 bounded on all sides by the germ -wall (Fig. 13, C, F). The 

 merging of the cells of the entoderm with the yolk mass is 

 shown in the small area of the germ wall drawn to a high mag- 

 nification in Figure 13, E. In the germ wall cell boundaries 

 are incomplete and very difficult to distinguish but nuclei can 

 be made out surrounded by more or less definite areas of cyto- 

 plasm. This cytoplasm contains numerous yolk granules in 

 various stages of absorption. It will be recalled that the nuclei 

 of the germ wall arise by division from the nuclei of cells lying 

 at the margins of the expanding blastoderm. They appear to 

 be concerned in breaking up the yolk in advance of the ento- 

 derm as it is spreading about the yolk sphere. 



Alxuit tin- twenty-second hour of incubation indications can 



