DEVELOPMENT OF THE SKULL OF EMYS 727 



which extends with its anterior apex ventral to the pila prootica 

 (g.s., fig. 8). In Emys the ganglia of the three branches of the 

 n. trigeminus are consolidated into one mass unlike the condition 

 in Lacerta where that of the ophthalmic branch is separated from 

 that of the other two and lies quite far anteriorly. 



The fenestra metoptica, through which the nn. oculomotorius 

 and trochlearis leave the cranial cavity, is a narrow slitlike open- 

 ing, having an oblique position, sloping forward and dorsally 

 from its base. It is bounded posteriorly by the pila prootica, 

 and anteriorly by the subiculum infundibuli, pila metoptica, and 

 planum supraseptale. From its anterior side the fenestra is 

 narrowed by the prolonged postero-lateral angle of the planum 

 supraseptale and again by a short posteriorly directed process 

 midway between this process and the lateral end of the pila metop- 

 tica. On the right side of the embryo modelled this second proc- 

 ess is reduced in the median part of its length so that only a proxi- 

 mal stump remains projecting from the posterior margin of the 

 planum supraseptale and a small isolated rod of cartilage which 

 lies freely within the fenestra (fig. 24). In another embryo of 

 approximately the same age, the fenestra metoptica becomes 

 regularly wider laterally and is narrowed only by the single 

 posteriorly directed process of the postero-lateral angle of the 

 planum supraseptale. The fenestra metoptica is not fully closed 

 dorsally because of the absence of a complete taenia marginalis, 

 although, as already described, the projection from the front 

 margin of the pila prootica and that from the hind margin of 

 the planum supraseptale suggest together an incomplete taenia 

 marginalis. 



The fenestra optica (f.opt., fig. 24) is of irregular triangular 

 form, somewhat broader than long. The planes of the two fora- 

 mina are slightly inclined toward each other in accordance with 

 the obtuse angle made by the two halves of the planum supra- 

 septale. The two foramina are separated from each other in the 

 middle line only by the free dorsal margin of the septum inter- 

 orbitale. Posteriorly they are bounded by the subiculum infun- 

 dibuli, and for a very short distance postero-laterally, by the pila 

 metoptica; anteriorly they are bounded by the planum supra- 



