38 



EMBKYONIC AXIS 



mes 3 



FIG. 55. TKANSVEBSE SECTION THROUGH AN EARLY HUMAN EMBRYO OF (H MM. 

 (Graf v. Spee ; cf. fig. 53.) 



am, aninion ; Dies' 1 , mesoderm of amnion ; ect, embryonic ectoderm; ent, entoderm ; mes 2 , meso- 

 derm of yolk-sac ; mes*, scattered cells between ectoderm and entoderm of germinal disc. 



V., 

 a 



FIG. 56. SURFACE VIEWS OF GERMINAL DISC OF TARSIUS SPECTRUM AT THREE SUCCESSIVE 

 STAGES. (After Hubrecht.) 



In a the dark spot represents Hensen's knot (protochordal knot, Hubrecht). As the pointed end of 

 the disc extends backwards, the thickening known as the primitive streak develops on it, c. 



