58 MR VV. E. AGAR ON THE DEVELOPMENT OF THE 



Growing out from the end of the internasal septum, slightly ventral to the cornua 

 trabecularum, is the prenasal process, which first appears about this stage. 



The opercular apparatus is particularly interesting at this stage. The hyoid arch 

 is sheathed in bone except at the symphysis and on the posterior face of the proximal 

 (vertical) portion, where the cartilage remains exposed throughout life. Intimately 

 attached to it at this point — in a stage earlier there is no definite line of demarcation 

 between the two — is a narrow plate of connective tissue which runs backward into the 

 opercular fold. In this plate (some distance from the hyoid) chondrification is taking 

 place — the rudiment of the interopercular cartilage found on the inner surface of the 

 bone in the adult (Bridge). The interopercular bone has already appeared. On the 

 inner side of the posterior end of the opercular bone (which was already present in 

 stage 34) there is another patch of connective tissue in which chondrification is pro- 

 ceeding — the rudiment of the cartilaginous nodule found in this place in the adult. 

 Both these strands of connective tissue fade away behind into the general connective 

 tissue of the opercular fold. To finish the account of these structures we may take 

 them up again in stage 38, fig. 16, in which they are practically in the adult condition. 

 The operculum consists of a nearly horizontal strip of bone attached in front by connec- 

 tive tissue to the lower edge of the squamosal, and ending freely behind. Here, to its 

 inner side, is attached the cartilaginous nodule mentioned above. The interopercular 

 cartilage is a long rod attached to the angle of the hyoid by a pad of connective tissue. 

 It is invested by bone (the interoperculum) on its outer side. This bone, though it 

 does not appear so soon as the operculum, nevertheless like it is laid down before the 

 cartilage underlying it. 



The facts of the development of these parts are in favour of Huxley's * suggestion 

 for Ceratodus and Bridge's for the Dipneumona, that the cartilages in connection with 

 the operculum and interoperculum represent vestigial hyoidean branchiostegal rays. The 

 stoutness of the interopercular ray is comparable to the great development of the 

 hyomandibular ray in Polypterus, which also persists as a nodule of cartilage on the 

 inner side of the opercular bone in this form (Budgett).I' 



All the bones have by now appeared. The parasphenoid now fills up the whole of 

 the basicranial fontanelle, and has lost all trace of its paired origin. The -palato -ptery- 

 goid had already attained its characteristic features in stage 34 (figs. 8 and 14). The 

 symphysis, however, is now more massive and pushes up the anterior ends of the 

 trabeculse at a sharper angle than before {cf. figs. 2 and 4). 



The splenial has completed its growth backwards, and instead of being confined to 

 the inner side of Meckel's cartilage, now arches over the top and partly covers it over 

 on the outside, leaving, however, a strip of cartilage exposed along the ventral outer 

 edge of the mandible (fig. 16). At its posterior end this strip is overlaid by a small 

 splint of bone, the angular. 



We left the dermal ectethmoid as a short rod of bone on the end of the ascending 



* Proc. Zool. Soc. Lond., 1876. t Trans. Zool. Soc. Lond., vol. xvi. part vii., 190*2. 



