314 ORR. - [Vol. I. 



close contact with the hypoblast below it, and with the medulla 

 above it. The dorsal wall of the head intestine, with its accom- 

 panying notochord, follows, therefore, the curved ventral surface 

 of the primary mid- and fore-brain around to that point imme- 

 diately ventral to the anterior medullary fold, where the hypo- 

 blast touches and fuses with the epiblast. At the base of the 

 anterior medullary fold the notochord runs into a mass of cells 

 {HC) which is continuous with the fused hypoblast and epi- 

 blast. The mass of cells seems, however, to be of hypoblastic 

 origin, in like manner as the notochord (Figs, ii, A-15, A, 

 PI. XIII.). 



The ventral wall of the head intestine curves slightly down- 

 wards to meet the epiblast at the point of the oral fusion. In 

 the present stage there are no traces of the branchial system : 

 when the branchial clefts appear later, the posterior clefts are 

 parallel to each other, and at right angles to the axis of the 

 intestine. The two anterior clefts are not parallel, but their 

 ventral ends are convergent. These facts seem to me explicable 

 only as the result of a general ventral bend of the anterior end 

 of the embryo, — a bend which appears primarily as the cranial 

 flexure, and imparts itself secondarily to the notochord and 

 head intestine ; for, owing to the relative mass of the different 

 parts, it would be impossible to suppose the converse method to 

 be true. 



We cannot avoid regarding as dorsal all that part of the 

 intestine which gives rise to notochord. In this case the noto- 

 chord extends as far as the oral fusion of hypoblast and epiblast; 

 therefore this point of fusion must be regarded as the extreme 

 anterior end of the head intestine. Owing to the curve of the 

 dorsal wall of the intestine, its lumen appears in section as far 

 forward as section 13, A, PI. XIII. ; but this is simply the dorsal 

 part of the intestine, not a " pre-oral intestine," for this latter, in 

 Anolis at least, would be impossible. 



Diagram I. will make the relation of these parts more readily 

 apparent. It will be seen that the fusion of notochord and 

 hypoblast with the epiblast at the pit of the " mouth involution " 

 appears to have a restraining influence on the epiblast, and to 

 hold it in place at this point. The external or ventral wall of 

 the " mouth involution" is simply the median part of the amnion, 

 and arises directly on the continuation of the lateral line of the 



