INGALLS. 



PLATE 2. 



Photographs of sections of axial structures, the locatirn ,unl direction of which are indicated 

 in the text-Inures. In all the photographs and drawings the right side of the embryo 

 is on the left in the plates, i. e. all views looking caudad. Dorsal structures are cut 

 ^liuhtly more anterior than ventral ones. (Cf. text-figure 3.) 



1. Section 42.3. Posterior part of primitive streak. X KiO. 



Section 406. Most caudal section in which arehenteric canal appears. Its location maybe 

 recognized by the absence of nuclei. X 160. 



:!. Section 401. Arehenteric canal where largest and best defined. X 400. 



I. Section*):). Large (second) ventral opening, "plaque notochordale" very distinct. X 400. 



:>. Section 380. Completion plate where best developed. X 160. 



(i. Section 375. Completion plate near anterior limit. X 160. 



