372 ROBERT J. TERRY 



gin of the parachordal plate. In the cat, the dorsum sellae is 

 also in part at least, derived from it, but probably the greater 

 part is derived from the mesenchyma above the crista transversa 

 and in front of the end of the notochord, in which the small 

 cartilaginous nodule was found in the smaller embryos. 



Let us consider briefly some observations regarding the dor- 

 sum sellae. I believe no comparative study of its formation 

 has been made. This structure reaches a high development in 

 man and apes. Fischer ('03) found in a Macacus embryo the 

 upper border isolated from the rest of the dorsum sellae, lying as 

 a transverse bar, and terminating laterally in the posterior cli- 

 noid processes. This author also points out that in the adult 

 human cranium a groove or ridge stands between the dorsal 

 border and the clivus and puts forth the suggestion that this 

 structure, isolated in Macacus, is probably genetically foreign 

 to the basal plate. Voit ('09) supports this view by the discov- 

 ery in Lepus of a partial separation of the dorsal part of the sad- 

 dle-back by a wide foramen from the rest of that structure. Faw^ 

 cett ('10) found in human embryos of 19 and 21 mm. a rounded 

 mass of cartilage, behind the pituitary body, connected with the 

 clivus region by a fibrous bridge; he concluded that the dorsum 

 sellae arises independently in man. In his study of the primordial 

 cranium of Talpa, Fischer ('01) found the hypophyseal fessa to be 

 a slight depression and noted the absence of a dorsum ephippii. 

 Noordenbos ('05) speaks of a weakly developed dorsum sellae 

 turcicae in mole embryos and states that it is a structure of the 

 parachordal plate. This investigator observed that in embryos 

 of 11 mm. the caudal end of the polar plate, lay somewhat be- 

 neath the cranial end of the parachordal plate. With the union 

 of these parts the hypophyseal fossa is formed, limited posteri- 

 orly by the projecting anterior end of the parachordal plate. 



Fischer's and Voit's observations point to the origin of the up- 

 per part of the dorsum sellae as possibly distinct from the basal 

 plate in Macacus and Lepus, and, according to Fawcett, the ori- 

 gin of the dorsum sellae in man is from an independent center of 

 chondrification. Noordenbos found the dorsum sellae of Talpa 

 as a product of the parachordal (basal) plate. These, observa- 



