396 OUTLINES OF CHORDATE DEVELOPMENT 



as in the chick, is in its origin more closely associated with the 

 ectoderm than with endoderm. Sections through Hensen's 

 node show that this is a region of thickened ectoderm and 

 mesoderm, less clearly differentiated from one another than in 

 the primitive streak region. The endoderm, however, is not 

 fused with the other cells, as it is in the chick. And in front 



Ce 



B 



FIG. 159. A. Transverse section through the primitive streak of the mole. 

 B. Transverse section through a human embryo of 1.54 mm. (Graf von Spec's 

 Embryo Gle.) From Minot (Laboratory Text-book of Embryology, after Heape 

 (A), and Graf von Spee (B). ch, Notochord; ct, somatic mesoderm; df, splanch- 

 nic mesoderm; EC, ek, ectoderm; en, En, endoderm;/, dorsal furrow; g, junction 

 of extra-embryonic somatic and splanchnic mesoderm; me, mes, mesoderm; p, 

 rudiment of embryonic ccelom; p,gr., primitive groove; Pr, primitive streak. 



of Hensen's node, in the region of the head process, mesoderm 

 is present, though entirely separate from the ectoderm, indicat- 

 ing its origin from the region of Hensen's node and not directly 

 from the ectoderm of the head process region. 



Later on conditions are found which make possible very close 

 comparison between the primitive streak and associated struc- 

 tures of the Mammal with those of the chick. Within the head 

 process, or in the region of Hensen's node, a rearrangement of 



