PRIMORDIAL CRANIUM OF THE CAT 329 



angle of the ala orbitalis is directed medially toward the trabecu- 

 lar plate and its two processes, preoptic and metoptic. The ex- 

 tremity of the latter presents the relation with the oculomotor 

 nerve observed in the earlier stage. From the metoptic process 

 a thick projection extends a short distance laterally, just behind 

 the optic nerve, to end in the muscle mass about the nerve; this 

 is the beginning of the orbital process of later stages. 



Van Wijhe preparations of embryos of 17 mm. (fig. 8) show a 

 lightly stained curved bar, the combined processus alaris and ali- 

 cochlear commissure, extending around the carotid artery, from 

 the side of the sella turcica to the anterior pole of the cochlear 

 capsule. Just in front and laterad of the commissure, and sepa- 

 rated by an interval of unstained tissue, is the triangular ala tem- 

 poralis, with the maxillary nerve against its anterior, concave 

 margin, and the mandibular nerve at its posterior side. The 

 separate cartilage behind the infundibulum in the preceding stage 

 is no longer apparent in the sections, but there is now at this 

 spot a dorsal, median process, partly cartilaginous, largely mesen- 

 chymal, of the crista transversa. Union of the orbital plate 

 with neighboring skeletal parts has occurred (fig. 8) : the medial 

 angle, presenting a notch for the optic nerve, is connected with the 

 pre- and metoptic processes of the trabecular plate; the anterior 

 angle is prolonged to the paranasal cartilage as the commissura 

 spheno-ethmoidalis, thereby completing the boundaries of the 

 fissura orbito-nasalis ; the posterior angle is fused with the com- 

 missura orbito-parietalis. This commissure, which has united 

 also with the parietal plate, has extended ventrally to the pars 

 canalicularis (commissura parieto-capsularis), but, as sections 

 show, is separated from the otic capsule by a thin layer of mesen- 

 chyma (p. 313). Continuity of the cartilage of the orbito-parie- 

 tal commissure and suprafacial commissure has been described 

 above (p. 301). 



Ethmoidal region. E7nbryo of 23.1 mm. . 



The parts of the chondrocranium included in this most clearly 

 defined region of the skull are the paired nasal capsules (figs. 1, 

 2, 3, 4). These conform closely with the membranous walls of 



JOURNAL OP MORPHOLOGY, VOL. 29, NO. 2 



