SKULL AND VISCERAL ARCHES IN LEPIDOSIREN AND PROTOPTERUS. 59 



process of the palato-pterygoid. In process of development the bone grows backward 

 in the fascia of the temporal muscle, keeping pace with the posterior extension of the 

 line of origin of this muscle. At first the muscle is circular in horizontal section,* but 

 as development proceeds the circle becomes more and more pulled out antero-posteriorly 

 as the muscle spreads backwards, to reach ultimately nearly to the hind end of the 

 auditory capsule. The posterior end of the dermal ectethmoid keeps a constant distance 

 in front of the posterior limit of the muscle. This relation to the muscle in development 

 is in favour of Wiedersheim's identification of this as a tendon bone. The anterior 

 end of the bone never has the intimate relations with the nasal capsule described by 

 Bridge for its homologue in Ceratodus, being from the first separated from it by 

 the processus ascendens of the palato-pterygoid. 



The dermal ethmoid, has appeared, investing the cartilaginous roof of the nasal 

 capsules. 



The squamosal is applied to the outer surface of the quadrate. The internal man- 

 dibular branch of vii. runs between the cartilage and the bone. 



The fronto-parietal is the last bone to appear. In stage 36 + the only calcification 

 is in the descending processes which overlie the upper part of the quadrate cartilage. 

 At this stage, therefore, the bone is paired. The connective tissue connecting the two 

 first-formed plates becomes ossified by the meeting of these plates in stage 37. There 

 is, therefore, nothing in the development of this bone which supports K. Furbringer's 

 scheme for homologising it with the sclero-parietal of Ceratodus. The fronto-parietal 

 was not present in the oldest Protopterus larva (stage 36). 



All the above-mentioned bones are membrane bones. The bone forming the sheath 

 round the hyoid arch, round the occipital arch (the pleuro-occipital bones), and round 

 the occipital rib is, however, deposited in the perichondrium, in direct contact with 

 the cartilage. These are therefore ectochondral bones. Endochondral ossification, as 

 is well known, does not take place. 



In stage 38, fig. 16, the skull has nearly reached its adult proportions. The chief 

 advance made by the chondrocranium over stage 36 + is the meeting in middle line of 

 the occipital arches to form the supra-occipital cartilage. This cartilage is produced 

 posteriorly in the middle line beneath the fronto-parietal, and in front extends a short 

 distance along the dorsal edges of the auditory capsules. From the outer edge of the 

 otic process of the quadrate a shelf of cartilage is growing out, underlying the squamosal 

 bone. Underneath the foramen pro-oticum this forms a cartilaginous loop enclosing t 

 " Pinkus's organ." (This loop is present in stage 36 + , but not visible in a dorsal view.) 



Fig. 16 shows the nodules of cartilage which have been supposed to represent 

 lower labials. Both these cartilages develop independently from Meckel's cartilage, just 

 as K. Furbringer found in a young Protopterus. The anterior nodule develops before 

 the posterior one, and about this stage becomes connected with Meckel's cartilage. The 

 posterior one remains separate throughout life. 



* Gf. fig. 10. t Agar, Anat. Anz., Bd. xxviii., 1906. 



