DEVELOPMENT OF THE SKELETON OF THE TUATARA. 



43 



region, so as to enclose the pituitary foramen, now undergrown by the parasphenoid 

 (cf. transverse section, PI. IV. fig. 3). The basi-occipital (b.o.) 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 a . 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. 7. Fig. 8. Fig. 9. 



n.xz? 



TtSS 



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 S, x 33. 9. Lateral longitudinal section at Stage Q, x 67. 10. Transverse 

 section at Stage R, x 33. 



b.o., basioccpital ; e.o., exoccipital ; Z.s., sustentacular ligament ; Z.s.', its exoccipital support ; md., medulla 

 oblongata ; we., notochord ; n. xii., 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 (n. xii., text-figs. 7 and 8), 



G2 



