54 MR W. E. AGAR ON THE DEVELOPMENT OF THE 



notochord, and the anterior ones penetrate the connective tissue connecting the Balken- 

 platten and occipital plates. On the chondrification of this tissue these parts of the 

 myomeres completely disappear, while a small artery, given off from the root of the 

 aorta and entering the cranial cavity, is enclosed by the cartilage (figs. 8 and 9). 



The dorsal end of the occipital arch is not yet fused with the auditory capsule, but 

 is attached to it by a strand of prechondral tissue. This strand is really a backward 

 continuation of the auditory capsule. Into it projects the posterior vertical semi- 

 circular canal. In progress of development chondrification proceeds from the auditory 

 capsule back along this strand, which at the same time increases in vertical extent and 

 receives more and more of the membranous labyrinth. In fact, the whole auditory 

 capsule is growing backwards. 



The cranial or occipital rib, i.e., the costal element corresponding to the occipital 

 arch, is now present. Histogenesis in this structure proceeds from the free to the 

 articular end. The head of the rib is situated slightly in front of the base of the 

 occipital arch in Lepidosiren (fig. 14), slightly behind it in Protopterus (fig. 15). No 

 other ribs are present yet, but these, when formed, are very much smaller than the 

 occipital ribs. 



The hinder ends of the Balkenplatten now approach much more nearly the notochord 

 than in the stage 31, and are only narrowly separated from the occipital plates. The 

 parachordal or basilar plate formed by the fusion of those cartilages strikingly resembles 

 that of the Urodeles. In Siredon (Stohr) # the first part of the skull rudiment to 

 appear are the trabeculse. Next the Balkenplatten develop, and quickly fuse with the 

 hind ends of the trabeculae. Next the occipital arch makes its appearance, then the 

 occipital plates grow forwards from their bases and fuse with the Balkenplatten. The 

 attachment of the auditory capsules to the Balkenplatten is by means of a mesotic 

 cartilage. The chief differences between the development of Siredon and Lepidosiren 

 or Protopterus are — 



1. The Balkenplatten in the latter, though they appear later than the trabeculse, 

 appear to chondrify in continuity with them. 



2. They are widely separated from the notochord at first. 



3. The mesotic cartilage is not a separate structure, in this point agreeing with the 

 majority of Vertebrates (Gaupp). The resemblance to the Urodeles is still greater in 

 Ceratodus (Sewertzoff). 



The Balkenplatten, trabecule, and auditory capsules form completely continuous 

 structures (fig. 8). There is no foramen ovale in the floor of the auditory capsule. 

 The side wall of the auditory capsule is continued for a short distance anteriorly as the 

 external wall of the posterior end of the Gasserian recess. The floor of this recess has 

 become greatly widened by the further development of the horizontal shelf shown in 

 figs. 7 and 13. The postero-external angle of this shelf has fused with the processus 

 oticus of the quadrate ; this is continued backward to the auditory capsule, or rather to 



* Zeitschr. fiir vms, Zool., 1880. 



