DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 



43 



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

 {of. transverse section, PI. IV. fig. o). The basi-occipital {h.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''. 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. 



Fis. 9. 





7l.S2^ 



nSS 



^ 



&.0 



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 E, x 33. 



h.o., basioccpital ; e.o., exoccipital ; l.s., sustentacular ligament ; l.s.\ its exoccipital support ; md., medulla 



oblongata ; nc., 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 



