DEVELOPMENT OF THE SKELETON OF THE TUATARA. 43 
region, so as to enclose the pituitary foramen, now undergrown by the parasphenoid 
(cf. transverse section, Pl. IV. fig. 3). The basi-occipital (0.0.) arises from a single 
ossific centre. The exoccipitals arise as ossifications of the occipital lobes of the chondro- 
cranium, and by extension reach the posterior boundary of the exit for the ninth to the 
eleventh cranial nerves. Osawa has recently described (98°. p. 494), with perfect 
accuracy, the adult condition of the nerve-foramina which result from this, and concern- 
ing the hypoglossal foramina he gives two as the number present. He also describes 
Fig. 8. 
Fig. 10. 
Figs. 7 to 10.—Sections through the occipital region of Sphenodon showing the hypoglossus nerve-foramina at 
different stages of development, and the sustentacular ligaments of the medulla. 7 and 8. Lateral 
longitudinal sections at Stage 8, x 33. 9. Lateral longitudinal section at Stage Q, x67. 10. Transverse 
section at Stage R, x 33. 
b.0., basiocepital ; ¢.0., exoccipital ; U.s., sustentacular ligament ; J.s.’, its exoccipital support; md., medulla 
oblongata ; n¢., notochord; n. wiz., hypoglossal nerve-roots and foramina. 
two nerve-roots. Gaupp accords a third hypoglossal foramen to the embryo Lacertilian 
(Berichte, p. 5). Interesting this, in consideration of modern discovery concerning 
the truncal origin of the hypoglossal nerve-bearing region of the skull. Still more 
interesting when we record the fact that, whereas in Sphenodon, at Stage S, three hypo- 
glossal foramina are present, with corresponding nerve-roots (w. avz., text-figs. 7 and 8), 
G 2 
