18 ILLINOIS BIOLOGICAL MONOGRAPHS [18 



arises from paired cartilaginous masses lying lateral to the notochord. Stohr 

 (1882) differentiated each of these masses into an anterior and a posterior part, 

 the anterior lying medial to the otic capsules, and the posterior behind the 

 exit of the vagus nerve from the cranium. The parachordal masses, as these 

 cartilages are called, eventually fuse, partially at least, with each other around 

 the notochord, anteriorly with the posterior ends of the trabeculae, laterally 

 with the otic capsules, and posteriorly form the base of the occipital region. 

 Concerning the parachordals in a general way, Parker and Bettany (1877; 

 p. 311) say: ''When the parachordals unite in the region where the notochord*^ 

 still persists, it is by growth of the cartilage over and under it. The bridge 

 beneath the notochord is very marked and becomes thick; the cartilage is 

 thinner above, and often nonexistent for a long time, so that the notochord 

 lies in a groove on the basilar plate constituted by the union of the parachordal 

 lia. In many cases where a basicranial fontanelle exists, the cartilages do not 

 approach one another again, and the fontanelle is only closed by a bony 

 growth. . . . The whole of the cranial notochord is gradually aborted in 

 most instances, and its place is occupied by cartilage; but in various forms a 

 remnant is left as a slender string, embedded in the basioccipital bone or 

 cartilage. " 



In the chondrocranium of the 10 mm. Amiurus, the parachordalia have 

 already passed through the early stages of development and are partially 

 fused with each other, with the trabeculae, and the otic capsules, forming the 

 base of the occipital region (Fig. 21). 



Terry (1917) has recently worked over the literature on the parachordal 

 region of the mammals and concludes that the parachordals may arise in three 

 ways : from a hypochordal center of chondrification ; from a pair of bilaterally 

 placed masses; and by growth and fusion of the apposed ends of the lateral 

 occipital arches. In the 10 mm. cat, the notochord enters the occipital region 

 between two laterally lying parachordal cartilages, dorsal to a mesenchymal 

 sheet which connects them. This sheet later becomes chondrified in connexion 

 with the parachordals, forming thus an hypochordal bridge of cartilage. This 

 agrees with the statement quoted from Parker and Bettany, but cannot be 

 apphed as a rule for the development of the basal plate of the teleost chondro- 

 cranium, as the condition in Amiurus shows. 



The concavity marking the anterior extent of the notochord on the ventral 

 surface of the plate is continued anteriorly beyond the tip of the notochord 

 as far as the margin of the fenestra basicranii anterior. The anterior end of 

 the notochord does not project into fenestra basicranii posterior, for such is 

 absent in Amiurus. There is no fenestra between the parachordal and the 

 otic capsule corresponding to the basicapsular fenestra of Salmo. / 



The sacculi of the inner ears have invaded the cartilage of the basal plate ^ 

 to such an extent that they have replaced most of it (Fig. 8). The grooves 

 on the dorsal surface of the plate containing them extend from below the base 



