No. 2.] EMBRYOLOGY OF THE LIZARD. 313 



fused together in only a short part of the middle dorsal region. 

 Toward the tail the lumen of the central canal gradually en- 

 larges, and finally is wide open above. Anteriorly to the apex 

 of the cranial flexure the lateral medullary folds spread wide 

 apart (^MF, Figs. 8, A-12, A), but in the most anterior part of 

 the head they curve toward each other and unite in a median 

 anterior fold (^i^ Fig. 13, A). By measurements of its 

 thickness and distance from the dorsal crest of the hind-brain, 

 it may be seen that this anterior fold is not a simple continuation 

 of the ventral floor of the primary first ventricle, but an elevated 

 fold continuous with the lateral folds, and enclosing the primary 

 first ventricle anteriorly as the lateral folds enclose it laterally. 



The epiblast, which is continuous with the dorsal edges of the 

 medullary folds, is spread a certain distance laterally over the 

 body of the embryo, and is then folded up over itself to form 

 the amnion. The line of this fold runs parallel to the curved 

 axis of the body and head, so that the line of the fold at the side 

 of the head curves ventrally and slightly backward. In Fig. 



13, A, the curved line of the fold is cut in two places on the left 

 side of the head. The part of the amnion springing from the 

 ventrally and backward curved line of the fold covers the mid- 

 and fore-brain. The epiblast, continuous with the dorsal edge of 

 the anterior medullary fold, is spread over the external surface 

 of the anterior fold until just beyond the ventral edge of the 

 latter it meets the line of the amnion fold, and recurving on 

 itself it forms the anterior median part of the amnion (see Fig. 



14, A, and its explanation). In a median sagittal section, the 

 anterior part of the amnion cavity appears to curve around 

 the head, and terminate with a small blunt end at that point 

 ■where the mouth will first appear, — i.e., the pit of the so-called 

 mouth-involution (cf. Diagram I.). These relations of the 

 amnion I have deduced by reconstructing the drawings of 

 consecutive transverse sections. 



The hypoblast extends as a blind sack into the head, forming 

 the rudiment of the head intestine. Along the dorsal median 

 area of the hypoblast arises the notochord : its origin from the 

 hypoblast has been described by Hoffman. In the embryo of 

 series A, the greater part of the notochord is distinctly de- 

 veloped, but in some places it still shows a more intimate con- 

 nection with the hypoblast. At this stage it lies everywhere in 



