72 



THE FIRST DAT. 

 Fio. 25. 



[CHAP. 



Four Transverse Sections through the Nedrenteric 

 Passage and Adjoining Parts in a Duck Embryo 



WITH TWBNTT-SIX MeSOBLASTC SOMITES. 



A. Section in front of the neurenteric canal, shewing a lumen 

 in the notochord. 



B. Section through the passage from the medullary canal 

 into the notochord. 



C. Section shewing the hypoblastic opening of the neuren- 

 teric canal, and the groove on the surface of the primitive streak, 

 which opens in front into the medullary canal. 



D. Primitive streak immediately behind the opening of the 

 neurenteric passage. 



mc. medullary canal ; ep. epiblast ; Ay. hypoblast ; ch. noto- 

 chord ; pr. primitive streak. 



