No. 2.] EMBRYOLOGY OF THE LIZARD. 3 i 5 



amnion fold, — indicated by the dotted line /, A, F. The ventral 

 curvature of the medullary folds gives to the " mouth involu- 

 tion " the appearance of being an invagination of epiblast to 

 meet the hypoblast; but its relation to the amnion fold does not 

 warrant such a conclusion. If we imagine the head continued 

 forward in a straight line (which was undoubtedly its more 



DIAGRAM I. 



AC- 



AC — Amnion cavity. AF — Anterior fold. AL — Alimentary canal. AM — 

 Amnion. II T — Heart. Hyp — Hypoblast. 1. A. F. — Lateral line of the amnion 

 fold. Min — Mouth involution. N — Notochord. NC — Neural canal. 



The dotted line running from the anterior fold to the apex of the cranial flexure 

 indicates the part of the brain-roof which is still open. 



primitive condition), then we should have the hypoblast of the 

 head intestine extending to the anterior surface of the head, and 

 fused with the epiblast at a point immediately ventral to the 

 medullary fold and just dorsal to the line of the amnion fold. 



Having passed the stage just described, and figured at i. A, 

 PL XII., the embryo begins to develop the various organs in 

 rapid succession, or some apparently simultaneously. Before 

 the primary first ventricle is completely closed anteriorly, the 

 optic outgrowths have made their appearance (Fig. 45, PI. XV.). 

 Shortly after the closing of the brain, the ear appears as a dorso- 

 lateral invagination of the epiblast. At the same time appear 

 the hypoblastic evaginations to form the gill-clefts. At this 

 time the mouth has not yet broken through (see series B). 

 Shortly after the appearance of the oral opening, the condition 

 of the head is as shown in the median sagittal section (Fig. 51, 

 D, PI. XV.). At this time the segmental organs are well de- 

 veloped ; the thyroid gland, the liver, and pancreas appear. Of 

 the outgrowths of the anterior half of the intestine, the thyroid 



