

III. 



V. 





FIG. 58. SEBIES OP TRANSVERSE SECTIONS THROUGH THE GERMINAL DISC OF TAHSIUS AT A STAGE 



SOMEWHAT EARLIER THAN THE DISC REPRESENTED IN FIG. 56, b. (After Hubrecht.) 



Section I. passes through the blastoderm in front of Hensen's (protochordal) knot, and shows only 

 the ectoderm and primitive entodermal plate. Section II. cuts the head end of the protochordal 

 process where it is continuous with the primitive entodermic plate. Sections III. and IV. pass through 

 Hensen's knot, which is seen in V. tapering away into the primitive streak. In III., IV., and V. the 

 mesoderm-sheets are seen springing from the keel-like thickening of the ectoderm, which in III. and 

 IV. is observed to be continuous into the entoderm. 



pr.st. 

 FIG. 59. TRANSVERSE SECTIONS THROUGH THE NEURENTERIC CANAL (NO i.) AND PRIMITIVE 



STREAK (NO. n.) OF A HUMAN EMBRYO OF 2 MM. (FIG. 72, P. 49). (After Graf v. Spee.) 



e ct, ectoderm ; mes, mesoderm ; ent, entoderm ; pr. st., primitive streak. The mesoderm-sheet 



springing from the streak show two lamellae. 



