PRIMORDIAL CRANIUM OF THE CAT 325 



The orbit (figs. 3, 4, 24, 25, 26) is relatively shallow and is ill- 

 defined in extent and boundaries. The planum antorbitale and 

 prominentia lateraUs of the ethmoidal skeleton form its anterior 

 wall; the ala orbitaUs and its two commissures enter into the 

 posterior wall. The dorsal hmit is given by the frontal bone 

 which extends along the spheno-ethmoidal commissure. The 

 ventral limits are found in the zygomatic and maxillary bones. 

 The ocular muscles arise in the following way. From the pre- 

 optic root springs the superior oblique; from the orbital process 

 of the metoptic root arise the rectus superior, internus and exter- 

 nus. The origin of the inferior rectus is conjoined with that of 

 the retractor oculi on the lateral surface of the cranial base, ven- 

 trad of the metoptic root and adjacent processus alaris; these 

 muscles arise within the spheno-orbital fissure. The inferior 

 oblique springs from the ventral margin of the planum antor- 

 bitale well forward of the origins of the other ocular muscles. 



Orhito-tem'poral region in smaller embryos 



Van Wijhe preparations of 10 mm. (fig. 5) present the first 

 trace of chondrification in the orbito-temporal region as a small 

 crescentic cartilage ventrad of the anterior hypophyseal lobe. 

 The concavity of the crescent embraces the hypophyseal stalk. 

 Behind this hypophyseal cartilage and occupying a dorsal plane 

 is the termination of the notochord. 



Sections of an embryo of 12 mm. (figs. 16, 17) show that the 

 cranial floor beneath the hypophysis consists of a horseshoe- 

 shaped cartilage independent of other parts of the chondrocra- 

 nium. The legs of the horseshoe lie on either side of the mid- 

 line beneath the anterior pituitary lobe, reaching as far forward 

 as the trabecular plate, but not united with it; the commissure 

 of the horseshoe crosses behind the stalk of Rathke's pouch. 

 Between the commissure and the terminal expansion of the basal 

 plate is a small space, fenestra basicranialis posterior, occupied 

 by mesenchyma in which the cephalic end of the notochord lies. 

 In median sagittal sections of embryos of 12 mm. (fig. 17) the 

 anterior expanded extremity of the basal plate is turned some- 

 what in the dorsal direction above the line of the posterior parts 



