OUTLINRS OF THE DEVELOPMENT OF THE TUATARA. 75 



groove in the floor of the alimentary canal (fig. 79, Th.) just 

 behind the mouth, lined by columnar cells. 



The Notochord. — The notochord appears at Stage E, 

 being formed by the separation of an axial rod of cells from the 

 thick sheet of mesoblast which grows forwards from the primi- 

 tive streak (figs. 26—33, 35, Not.). Thus the notochord in 

 Sphenodon appears to be undoubtedly of mesoblastic origin, 

 an origin which, it will be remembered, is also claimed for 

 it by some writers (e. g. Duval [11] ) in the chick. It extends 

 forwards to within a short distance of the infundibuluui, and 

 in the later stages of its development its anterior end becomes 

 curiously twisted in an irregular spiral (fig. 95, Stage N), which 

 is recognisable even after the cartilage has developed around it 

 (Stage R). 



The Ccelomic Cavities. — The first indication of coelomic 

 cavities appears extremely early, at Stage D, in the form of a 

 wide space in the mesoblast of the area pellucida in front of 

 and entirely outside the embryo itself (fig. 8, P. C). This 

 space appears to be the rudiment of the pericardial cavity 

 which is so conspicuous in later stages, but which is not for a 

 long time shut off" from the general body-cavity of the adult. 

 As the head-fold deepens the coelomic space in question is 

 carried back, and comes to lie just in front of the opening into 

 the anterior enclosed part of the alimentary canal (fig. 50, 

 P. C). It is possible that it does not always appear so early 

 as Stage D, for in embryo 61 (Stage F, fig. 35) I observed no 

 trace of it, and at Stage G (fig. 50) it appears as a space 

 between epiblast and hypoblast, with no mesoblast around it. 

 Possibly it normally appears very early, and is then more or 

 less completely obliterated, perhaps by stretching in the forma- 

 tion of the pro-amnion, to open out again later on. Already 

 at Stage H, however, it forms a conspicuous quadrangular sac 

 containing the heart, lying beneath the anterior enclosed part 

 of the alimentary canal (fig. 52). 



Sections of embryos of Stage J, in vvhich the pericardial 

 cavity is even more conspicuous (fig. 59), clearly shovr that it 

 is in free communication with the general body-cavity above. 



