SKULL AND VISCERAL ARCHES JN LEPIDOSIREN AND PROTOPTERUS. 51 



of the membraneous tissue of the previous stage into cartilage is beginning. In the 

 figure only those parts of the cranium marked out by the commencement of this process 

 are shown. Consequently the outlines are somewhat arbitrary at the hind end of the 

 figure. The figures, however, show all those parts of the skull which have arrived at 

 the same stage of histogenesis. 



The front end of each trabecula now takes the form of a somewhat thin lamina of 

 prechondral tissue, inflected at its lower margin, following the contour of the 

 thalamencephalon. The optic nerve runs in front of the abrupt edge of this lamina. 

 At its extreme anterior end each trabecula is produced into a flattened dorsal spine, 

 on the outer side of which runs the ophthalmicus profundus branch of the fifth nerve* 

 (v. 1 , figs. 7 and 13). For the sake of convenience this spine may be called the orbito- 

 temporal process. Close behind this each trabecula is provided with a triangular 

 horizontal shelf projecting externally. This is the floor of the front end of the 

 Gasserian recess (the name given by Bridge f to the cartilaginous recess lodging the 

 very large ganglionic mass representing the ganglia of v., vii., and vii. lateralis). 

 Behind this shelf two nerve trunks pass over the trabecula — the superior maxillary 

 branch of the fifth nerve (v. 2 , figs. 1 and 5), and a composite nerve consisting of the 

 inferior maxillary branch of the fifth, and the buccal and superficial ophthalmic branches 

 of the seventh nerve (v. 3 , vii. lat). 



The hinder part of the skull rudiment on each side extends further posteriorly than 

 in the previous stage, the new part representing the Balhenplatte (Stohr) portion of 

 the parachordal cartilage. The wide separation of the Balkenplatte of each side from 

 the notochord is noteworthy. The position of the auditory capsules is indicated in 

 fig. 7 in section. They at present show hardly any signs of chondrification. There 

 is at present no indication of the occipital portion of the parachordals. 



The rudiment of the quadrate cartilage is from the first continuous with the trabecula 

 by a process which the disposition of the nerves shows to be the Processus basalis. In 

 this respect the Dipneumona contrast with the more primitive condition found by 

 Sewertzoff \ in Ceratodus, where the quadrate is at first free from the trabecula. The 

 hyoideo-mandibular nerve runs ventralwards behind the basal process (figs. 7, 13, vii. 

 hyo.). Separated from the quadrate by a region of more embryonic tissue is Meckel's 

 cartilage. This does not yet meet its fellow in the middle line, nor does the hyoid 

 arch. The latter shows no signs of segmentation at this or any other stage, nor was 

 the hyo-mandibular, found by Sewertzoff (loc. cit.) in Ceratodus and confirmed by 

 K. Furbringer, § observed at any stage. 



A strand of dense connective tissue, connecting the distal end of the rudiment of the 

 quadrate with that of the palatine tooth, probably represents the vestige of the palato- 

 pterygoid cartilage (fig. 13). At its hind end it passes directly into the substance of the 



* Pinkus, "Die Hirnnerven des Protopterus annectens," Morph. Ark, Bd. iv. 1895. 

 t Trans. Zool. Soc, 1898. t Anat. Anz. Bd. xxi., 1902. 



§ Beitrage zur Morphologie des Skeletes der Dipnoer, 1904. 



