DEVELOPMENT OF THE SKULL OF EMYS 699 



capsule. Along the dorso-median wall of the sulcus open the three 

 foramina spino-occipitalia and at its anterior end opens the ductus 

 perilymphaticus. The ganglia vagi and spino-occipitalia are situ- 

 ated lateral to the groove. 



The foramina spino-occipitalia and facialis lie rather in the 

 lateral portion of the cranium than in the basal plate, but they 

 may be conveniently described at this time. There is some 

 variation in the number and relations of the foramina spino-occi- 

 pitalia in the series of embryos studied, although they always lie 

 in two horizontal lines, which converge in front, and are ventral 

 to the bases of the arcus occipitales and fissura metotica. They 

 pierce the basal plate in a ventro-lateral direction and open exte- 

 riorly in the sulcus supracristularis. In the individual modelled 

 there are three pairs of foramina, of which the anterior is the 

 smallest and the posterior the largest. The middle foramen is 

 equally distant from the other two. In the adult, as is well 

 known, the first and second pairs of nerves leave the cranial cavity 

 through the same foramen, and only two pairs of foramina spino- 

 occipitalia are present. In two younger embryos having a cara- 

 pace length of from 7 to 8 mm. the adult condition was met with. 

 In one embryo, older than that modelled, the first and second 

 foramina were united at their external ends but completely sepa- 

 rated from each other internally. On the other hand in one of the 

 oldest individuals studied all three pairs of foramina were quite 

 distinct as in the model. 



The foramen facialis (/./., fig. 6) lies well in front of the anterior 

 end of the otic capsule and relatively far laterally in the side por- 

 tion of the otic region. Behind this foramen the basal plate is 

 continuous with the otic capsule for a long space. In front of 

 it there is only a slender commissure uniting the basal plate and 

 the cupula anterior of the capsule. 



The foramen abducentis (/.a., figs. 8 and 24) passes hori- 

 zontally forward at the anterior end of the crista sellaris, as a 

 moderately long canal opening anteriorly ventral to the pila 

 prootica and dorsal to the crista basipterygoidea, and poste- 

 riorly on the dorsal surface of the basal plate immediately behind 

 the proximal end of the pila prootica. 



