The Chondrocraniiim of an Embryo Pig. 199 



the primates (Macacus, Semnopitheciis and Homo) it is merely in- 

 dicated by a posteriorly-projecting point on the outer part of the ala. 

 Tarsius shows a stage intermediate between that of the primates 

 and of the other manunals, in that this posterior process is in the 

 form of a rod which extends almost to the ear-capsule. It forms 

 the dorsal border of the foramen spheno-parietale, the largest opening 

 in the side wall of the primordial cranium. 



The commissura orhitonasalis (cartilago spheno-ethmoidalis, or 

 commissura spheno-frontalis of Spondli) is a narrow bar connecting 

 the ala orbitalis with the dorso-lateral portion of the nasal capsule, 

 and enclosing beneath it and between the ala and the capsule the 

 large oval fissura orhitonasalis. This fissure, which is at this stage 

 filled mainly with undifferentiated tissue, is later nearly closed by 

 the backward migration of the nasal capsule and the forward growth 

 of the ala orbitalis, leaving only a small opening for the passage of 

 the nervus ethmoidalis. 



Regio Ethmoidalis. 



The nasal region of the embryonic skull approaches more closely 

 in form the corresponding region in the adult skull than does any 

 other portion of the chondrocranium. The chief difference is in 

 the excessive lengthening in the face of the adult, a condition devel- 

 oped mainly after birth. The shape of the nasal capsules as a whole 

 is that of two closely appressed cylinders, whose diameters are 

 greater in their middle portions than toward their ends, where they 

 taper almost to points. An oblique slice has been removed from their 

 postero-dorsal portions along the plane where the cribriform plate 

 is later developed. The total length of the nasal capsules in Sus em- 

 bryos at this stage is equal to 1/3 that of the entire skull, in Talpa em- 

 bryos it equals 2/5, while in those of Semnopithecus it is only equal 

 to about 1/5 of the cranial length. In this character the Sus embryo 

 occupies an intermediate position, showing neither excessive lengthen- 

 ing nor shortening. 



Let us turn our attention now to some of tlie details in the 

 different parts of the nasal capsules. 



Along the mid-dorsal line, in front of the foramina cribrosa and 



