THE EARLY DEVELOPMENT OF THE MAMMAL 397 



cells produces a small cavity known as the notochordal canal. 

 This varies greatly in extent. In such forms as the guinea-pig 

 and bat, the canal is comparatively long, extending from Hen- 

 sen's node, where it opens upon the surface by a definite per- 



FIG. 160. A. Sagittal section through the embryonic shield of the hedgehog, 

 showing the transitory blastopore. After Hubrecht. B. Posterior part of a 

 sagittal section through the embryonic disc of the mole. C. Diagram of a 

 aagittal section through the embryonic disc of the mole. From McMurrich 

 (Development of the Human Body), after Heape. am, Amnion; bl, blastopore; 

 ce, chorda endoderm; ec, ectoderm; en, endoderm; nc, neurenteric canal; prm, 

 prostomial mesoderm; ps, primitive streak; t, trophoderm. 



foration through the ectoderm, anteriorly and ventrally ; 

 finally either opening through the endoderm into the cavity 

 of the endodermal vesicle (Fig. 160, C), or simply ending blindly 



