344 EGBERT J. TERRY 



region of Froriep's occipital vertebra. Noordenbos ('05) found 

 the occipital basal anlage to be single in the mole, calf and 

 pig and also in the rabbit. Regarding the parachordal plate 

 in the latter, this author found that it was formed by the fusion 

 of the opposed ends of the two free occipital arches which were 

 first to develop and from this starting point, grew forward (p. 

 375) . Here are then, apparently, three different conditions pre- 

 sented in the origin of the cartilaginous basal part of the 

 occipital in manmaals: the appearance of a hypochordal center; a 

 pair of bilaterally placed masses; origin by growth and fusion 

 of the apposed ends of the lateral occipital arches. 



Regarding the relation of the notochord to basal plate, it was 

 found that in cat embryos of about 10 mm., the former enters 

 the occipital region between the parachordal cartilages and lies 

 in the dorsal part of the mesenchymal sheet which unites these 

 cartilages across the midplane. This mesenchymal sheet later 

 becomes chondrified in connection with the parachordals, form- 

 ing thus a hypochordal bridge closing the space which originally 

 separated these cartilages (Terry, '13). Where the notochord 

 lies between the parachordals it is surrounded by a layer of 

 denser mesenchyma than that concerned in hypochordal arch 

 formation; this specialized sheath is continuous with the mesen- 

 chyma about the notochord in the region of the atlas — that in 

 which the atlantal centrum is later formed. In regard to a 

 cartilaginous hypochordal layer in the occipital region of mam- 

 mals generally, there seems to be no doubt of its constant oc- 

 currence. My own observation of the position of the noto- 

 chord with reference to the basi-occipital region in cat is in agree- 

 ment with that of Williams ('08), namely, that after chondrifi- 

 cation is well established, the caudal portion of the basal plate 

 is hypochordal. Recently Kernan ('15) has observed the hypo- 

 chordal position of the basal plate in cat embryos. Parker, 

 Froriep, Levi, Noordenbos, and Weiss have observed, in various 

 mammals, that the caudal part of the occipital basal cartilage is 

 ventrad of the notochord. 



The parachordals in cat are united at their anterior ends 

 across the midplane (embryos of about 10 mm.) by a hypo- 



