34 EARLY EMBRYOLOGY OF THE CHICK 



13, a different conventional scheme of representation has been 

 employed to indicate each of the germ layers. The ectoderm is 

 vertically hatched, the cells of the mesoderm are represented 

 by heavy angular dots when they are isolated or by solid black 

 lines when they lie arranged in the form of compact layers, 

 and the entoderm is represented by fine stippling backed by a 

 single line. This same conventional representation of the 

 different germ layers is observed in all diagrams of sections in 



cephalic end 



anterior border 

 of mesoderm 



neural plate 



la 



embryonal area 



1^ '^r .-. -~rr Hensen's nod 



Ktf Y 'E-^-tfT" primitive streak 



'*.' 

 if-. 



^^HRT^ - ' 



caudal end 

 FIG. ii. Dorsal view ( X 14) of entire chick embryo of 18 hours incubation. 



order to facilitate following the way in which the organ systems 

 of the embryo are constructed from the germ layers. Details 

 of cell structure are for the most part omitted with the expecta- 

 tion that the student will acquire a knowledge of them in his 

 own study of sections. The plane in which each of the sections 

 diagrammed passes through the embryo is indicated by a line 

 drawn on a small outline sketch of an embryo of corresponding 

 stage. For interpretation these outline sketches should be 

 compared with actual specimens or detailed drawings of entire 

 embryos of the same stage of development. 



In embryos of the stage under consideration the relationship 

 of the germ layers at the primitive streak still indicates their man- 

 ner of derivation (Fig. 13, C and D). The ectoderm and the 



