OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 71 



medullary groove is widened out and constricted into fore-, 

 Tiiid-, and hind brains (figs. 20, 21), the hind brain already 

 exhibiting traces of further segmentation. At Stage F the 

 medullary groove begins to close in the region of the mid- 

 brain (figs. 35, 42). At Stage G the closure has extended 

 backwards and forwards, leaving the medullary groove open 

 above only in two places, viz. — posteriorly, above and in front 

 of the now conspicuous neurenteric aperture ; and anteriorly, 

 in the region of the fore-brain (fig. 50). The mid and hind 

 brains are now bent somewhat into the form of an S by the 

 backward and upward tilting of the head due to the restraining 

 influence of the proamnion (fig. 50), and, owing doubtless to 

 the same cause, the two halves of the fore-brain are beginning 

 to overlap one another in the middle line (fig. 48). At Stage 

 H the mid- and hind brains are straightened out again, but 

 the overlapping of the left half of the roof of the fore-brain is 

 more conspicuous, while the optic vesicles commence to grow 

 out on either side (fig. 52). At Stage G the medullary canal 

 is completely closed in above, though the left half of the fore- 

 brain still overlaps the right (fig. 60), and the margins of the 

 two do not actually unite until the next stage (fig. 77). At 

 Stage J, also, the fore-brain bends down at right angles, so 

 that the mid-brain comes to occupy the anterior end of the 

 body (figs. 58, 59), and the segmentation of the hind brain is 

 much more strongly marked (tig. 58). The roof of the hind 

 brain also is thinning out in the usual manner (fig. 63). At 

 Stage K the primary parietal vesicle is formed as an outgrowth 

 of the roof of the fore-brain a little to the left of the middle 

 line (fig. 76). At Stage L, by the closure of the neurenteric 

 canal, the central nervous system becomes completely shut in; 

 the cerebral hemispheres begin to grow out from the fore- 

 brain, and the infundibulum and pituitary invagination almost 

 meet one another beneath the thalamencephalon, while the 

 hind brain has become segmented into about six neuromeres 

 (fig. 90). At Stage M the mid-brain has become extremely 

 prominent, and the cerebellum begins to be conspicuous just 

 behind it (fig. 92). At Stage N the paraphysis appears as a 



