OUTLINES OP THE DEVELOPMENT OP THE TUATARA. 37 



wall of the aliinentary canal has already begun to thin out, 

 preparatory to perforation by the mouth. A short way behind 

 this is seen the heart (fig. 57, Ht.), lying in the pericardium, 

 a little in front of the middle of the body. Just behind the 

 heart lies the opening of the anterior enclosed part of the 

 alimentary canal. The remainder of the ventral surface is 

 occupied by the as yet unenclosed portion of the alimentary 

 canal in the form of a shallow trough with prominent margins, 

 and with the aperture of the neurenteric canal (fig. 57, N. En.) 

 lying near its posterior end. 



The Alimentary Canal. — The extreme anterior end of 

 the alimentary canal lies in the angle between the mid-brain 

 and the downwardly turned fore-brain, its blind extremity 

 being in the same transverse plane as the now posterior limit 

 of the fore-brain beneath it. Fig. 62 represents a transverse 

 section taken just behind the anterior extremity of the 

 alimentary canal, which in this region occupies only about 

 one third of the total width of the head. Beneath the 

 hind brain (fig. 63) it widens out until its side walls meet and 

 fuse with the lateral epiblast, preparatory to the formation of 

 the first visceral cleft. Its floor also here becomes extremely 

 thin, the epiblast of the stomodseum (fig. 63, Stom.) lying in 

 close contact with the hypoblast, and both being attenuated 

 preparatory to perforation by the mouth. On approaching the 

 pericardium, however (figs. 64, 65), the lateral and ventral 

 walls of the alimentary canal retreat from the epiblast and 

 assume their normal aspect, which is maintained until, on 

 reaching the opening to the exterior behind the heart, the 

 folds of the splanchnopleure diverge, and leave the alimentary 

 canal widely open below (fig. 67). This open condition is 

 continued to the posterior extremity behind the conspicuous 

 aperture of the neurenteric canal (fig. 70). The anterior 

 enclosed part of the alimentary canal is densely filled with 

 yolk particles, conspicuous both in sections (figs. 62 — 66), and 

 when the whole embryo is viewed as a transparent object 

 (fig. 59). Its hypoblastic lining is now composed almost 

 throughout of the characteristic short columnar cells, flattening 



