OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 67 



the primitive streak so as to give rise to a long narrow canal or 

 tunnel, the posterior amniotic canal, which for some time opens 

 on the surface of the blastoderm behind, and thus maintains 

 a free communication between the amniotic cavity above the 

 embryo and the very small space beneath the shell containing 

 the " white " of the egg (figs. 35, 37, 39, 40, 41, 45, 71—73, 

 P.A.C.). The posterior amniotic canal is formed partly by 

 invagination of a specially modified linear strip of epiblast, 

 and partly by uprising of two somatopleuric folds which meet 

 and unite above it (fig. 46). It makes its appearance at 

 Stage F, and disappears by obliteration of its lumen at about 

 Stage K. After the serous envelope has split off from the 

 underlying yolk-sac it lies entirely in the thickness of the 

 former, embedded in the mesoblast (figs. 71 — 73). 



So far as I am aware, the only other type in which anything 

 comparable to the posterior amniotic canal of the Tuatara has 

 been found is the Chelonian type, in which, according to 

 Mitsukuri, the amniotic folds continue to grow backwards, and 

 thus produce a tube which extends back from the posterior end 

 of the embryo and connects the amniotic sac with the exterior. 

 Although my knowledge of Mitsukuri's observations is derived 

 solely from the abstract in the ' Journal of the Royal Micro- 

 scopical Society^ (15), I suppose there can be little doubt as to 

 their close agreement with what I have myself observed in the 

 Tuatara, and we thus have a striking embryological confirma- 

 tion of the view so strongly insisted upon by Boulenger (16) 

 as to the close relationship of S phenodon with the Chelonians, 

 a view which has hitherto been based entirely upon the 

 structure of the adult animal. 



As to the functions of the posterior amniotic canal I have no 

 suggestions to make, but I understand that Mehnert has pub- 

 lished a paper (18) on the subject, to which, unfortunately, I 

 have been unable to obtain access. 



The Serous Envelope. — The serous envelope is formed 

 only to a very slight degree by the outer limb of the uprising 

 amnion folds. Throughout nearly its whole extent it is formed 

 by splitting ofi" of the superficial epiblast, accompanied by a 



