OUTLINKS OF THE DEVELOPMENT OF THE TUATARA. 41 



right and left sides are reversed^ again shows that it is the left 

 half of the roof of the fore-brain which overlaps the right. 

 Posteriorly, the medullary canal still communicates freely with 

 the as yet unenclosed portion of the alimentary canal through 

 the conspicuous neurenteric passage (fig. 70, N. En.). 



The fore-brain (figs. 59 — 61, F. B.), as already observed, is 

 bent down at right angles to the long axis of the body, and 

 presents two conspicuous hollow lateral outgrowths, the optic 

 vesicles of the ordinary paireil ryes (figs. 59 — 61, 0. V.). 



The mid-brain (figs. 58, 60, 61, M. B.), nearly spherical 

 when seen from above, occupies the extreme anterior end of 

 the body. It now lies in the same straight line with the hind 

 brain, from which it is separated by a very strongly marked 

 constriction, while the curvature which existed in this region 

 at preceding stages has disappeared. 



The hind brain (figs. 58, 63, 64, H.B.) shows indications of 

 transverse segmentation into at least four neuromeres, and its 

 roof is thinning out '\\\ the usual manner (fig. 63). It tapers 

 very gradually behind, and passes insensibly into the spinal 

 cord. 



On either side of the posterior part of the hind brain the 

 auditory pits (figs. 58, 64, Au.) have made their appearance as 

 shallow depressions of the superficial epiblast, widely open to 

 the exterior and lined by elongated columnar cells. 



Other Embryos. — Atnongst the other embryos which I 

 have referred to Stage J, the following seem worthy of special 

 mention. 



In No. 40 the posterior amniotic canal is still distinctly 

 visible in the unstained embryo when viewed as an opaque 

 object. It runs from the hinder end of the embryo obliquely 

 backwards, inclining towards the left, as in embryo 72 (Stage F, 

 figs. 37, 39), and opens to the exterior on the edge of the clear 

 area of the blastoderm. I have not detected this canal ex- 

 ternally in any other embryo of Stage J, though, as noted 

 above, it was easily demonstrable in sections of No. 44. 



' Tills overlaj) was not observed until the sections were cut, and is there- 

 lore not shown in Fi^r. 59. 



