212 OUTLINES OF CHORDATE DEVELOPMENT 



Immediately subsequent events concern chiefly the posterior 

 part of the cranium. The auditory organ becomes partially 

 enclosed by a connective tissue capsule which is early chon- 

 drified, forming a cap open toward the mid-line (Fig. 80, B). 

 A cartilage (mesotic cartilage) then extends posteriorly and 

 laterally from the parachordal plate and becomes united 

 with the auditory capsule by anterior and posterior ventral 



sr 



M 

 ct 





FIG. 81. Chondrocranium of 29 mm. larva of R. fusca. After Gaupp, from 

 Ziegler. To the left, the ventral surface; to the right, the dorsal surface, a, 

 Auditory capsule; bp, basal plate; c, notochord; ct, trabecular cornu; /, basi- 

 cranial fontanelle; fa, foramen for carotid artery; fm, foramen magnum; fo, 

 foramen for olfactory nerve; ir, infrarostral cartilage; j, jugular foramen for 

 IX and X cranial nerves; I, perilymphatic foramina; m, muscular process; M, 

 Meckel's cartilage; o, otic process of palato-quadrate; pf, palatine foramen; 

 pq, palato-quadrate cartilage; sr, suprarostral cartilage; t, trabecular cartilage; 

 v, secondary fenestra vestibuli. 



connections, leaving between them a wide space. Posteriorly 

 to this mesotic cartilage the floor of the cranium is continued 

 as the occipital cartilage. This also fuses with the floor of 

 the auditory capsule leaving, however, a small space which 

 represents the jugular foramen transmitting the IX and X 

 cranial nerves. The floor of the posterior part of the cranium, 



