322 ORR. [Vol. I. 



lenteric zone separates from the epiblast much earlier than does 

 the notochord, and disappears ; while the notochord remains a 

 long time connected with the epiblast or hypophysis. 



3. The Alimentary Canal. 



The alimentary canal in the youngest stage examined by me 

 (series A) has already been described. I need only add that 

 in the region of the mouth it appears laterally compressed, 

 while behind the mouth it is dorso-ventrally compressed. Be- 

 hind the region of the mouth the lumen appears everywhere 

 of equal size as far back as the yolk-sack. No traces of gill- 

 pouches have appeared. 



The oral fusion of epiblast and hypoblast appears very early 

 in the lizard. Hojfman has described it as present in an em- 

 bryo with only two somites. Balfonr has pictured the two 

 layers in contact in this region in an elasmobranch embryo, of 

 which the medullary groove is only a slight depression. It 

 seems probable, therefore, that in these cases no mesoblast ever 

 develops between the other two layers at the point where the 

 oral fusion appears. If this supposition be true, the mouth- 

 opening of the lizard presents a method of origin very different 

 from that of the gill-clefts. 



The gill-cleft rudiments first appear as paired pouch-like 

 protrusions from the dorso-lateral parts of the alimentary canal 

 (series B). They grow toward the epiblast, pushing through 

 the mesoblast, which at first entirely surrounded the alim.entary 

 canal. The first and second clefts are the first to acquire an 

 external opening. Then follow in order the third and fourth 

 clefts (Figs. 30, C, and 31, C, PI. XIV.). Behind the fourth 

 there appears later a fifth rudiment, for which alone I have 

 never detected any external opening. In longitudinal-horizon- 

 tal section there may be seen in some embryos small rounded 

 swellings of the epiblast on the lateral posterior corners of the 

 gill arches. These were at fi^rst suggestive of gills, but are per- 

 haps simply remnants of the breaking through of the epiblast. 

 That part of the alimentary canal from which the gill-clefts 

 open is, comparatively, extremely large, and may be supposed 

 to indicate its condition at the time when it was functionally 

 active, as part of the respiratory system. 



