PRIMORDIAL CRANIUM OF THE CAT 327 



ganglion. This center is located at the site of the anterior part 

 of the alar element. 



In sections the ala orbitaUs is a plate of condensed mesenchyma 

 Umited sharply in front, above and behind, but connected with 

 the trabecular plate by two roots. The preoptic root extends 

 from the trabecular plate anterior to the sulcus chiasmaticus and 

 passes laterally without interruption into the main body of the 

 ala. The metoptic root is joined to the side of the caudal por- 

 tion of the trabecular plate. This root terminates laterally in a 

 pointed extremity connected by less dense mesenchyma with the 

 ala orbitalis. The extremity lies beyond the optic foramen, im- 

 mediately dorsad of the third nerve where this enters the orbit. 



The cavum epiptericum in the 12 mm. embryo, is clearly de- 

 fined toward the base of the neuro-crailium. The membrana 

 Umitans (figs. 16, 17) can be followed over the medial aspect of 

 the semilunar ganglion to the anterior end of the basal plate, to 

 the conunissural element and over the carotid artery. Upon 

 the lateral side of the membrane, opposite the vessel runs the 

 abducent nerve. At its insertion into the anterior margin of the 

 basal plate (the future crista transversa) the limiting membrane 

 becomes continuous with the septum transversum. Within the 

 latter immediately caudad of the hypophysis is a small, trans- 

 versely placed precartilaginous rod (indicated by an * in fig. 17). 



The pterygoid cartilage is represented by a condensation of 

 mesenchyma lying anterior and ventral of the alar element and 

 close to the epithelium of the pharynx. A constriction incom- 

 pletely subdivides it into dorsal and ventral parts. Behind this 

 mass is the first pharyngeal pouch; through its dorsal extremity 

 runs the pterygoid nerve. 



- In embryos of 15 nun. the hypophyseal cartilage has united 

 anteriorly with the trabecular plate, forming the floor of the sella 

 turcica, perforated by a foramen hypophyseos (figs. 7, 18). 

 Union of the basal plate and hj^pophyseal cartilage has also oc- 

 curred, but the confines of the originally separate cartilages are 

 still evident (fig. 18). As a result of the incongruity in the meet- 

 ing of these parts (indicated in the 12 mm. stage) there remains 

 a transverse ridge in the basis cranii behind the pituitary fossa, 



