44 



GASTRULA THEORY 



III. - 



FIG. 66. A SERIES OP TRANSVERSE SECTIONS THROUGH THE NEURENTERIC CANAL OF A MOLE 



EMBRYO. (Heape.) 



The dorsal opening is shown in I., continued into the primitive groove ; the canal passes thence 

 through the head end of the primitive streak (II.) into the thickened posterior end of the notochord- 

 plate (III.), along which it extends for some distance (IV.), and eventually opens ventrally in a 

 median groove (V.). 



ec, (me, en, ectoderm, mesoderm, entoderm ; p.gr. (in I. and II.), primitive groove; c, neurenteric 

 canal ; m.gr. (in III., IV., and V.), medullary groove. 



