| OCR Text |
Show 586 MR. J. B. SUTTON ON THE DEVELOPMENT [June 2, passes towards the anterior limit of the fronto-nasal plate. Later rod becomes segmented as follows:-the distal end becomes internal pterygoid plate, the middle portion persists as the cartilaginous piece of the Eustachian tube, and the proximal portion degenerates into ligament. The details of the metamorphosis of this bar will be found in m y paper on the " Parasphenoids " &c. (Proc. Zool. Soc. November 1884). In the Crocodile the corresponding bar of cartilage is continuous posteriorly with the huge quadrate, its middle portion is considerably infringed by the so-called pterygoids, whilst its distal end is continuous with the os transversum. If the bones forming the posterior limit of the hard palate in Crocodiles are not to be regarded as pterygoids, to what do they correspond? They are the homologues of the avian basitemporals. As is the case with the so-called pterygoids of the Crocodile, the bird's basitemporals are preformed in membrane, they underlie the basisphenoid, and the Eustachian tubes lie above them; but the bird in its aerial mode of life needs not a long tubular nasal passage, indeed its hard palate may be considered defective, and each basitemporal, instead of sending a process of bone to curve around the* posterior nares, merely persist as a flat plate of bone which eventually becomes welded to the skull-base. In birds the pterygoids take a different direction from that of the Crocodile's os transversum : in the former case they converge anteriorly, and in some avian skulls actually come into contact at the spot where they join the palatines, whilst posteriorly they abut upon the quadrate bone ; this last fact is sufficient to prevent any misinterpretation as to their nature. In the Crocodile the anterior ends of the pterygoids are carried outwards, until they rest on the maxillae, and the postpalatine bones (the so-called pterygoids), being wedged in between them, separate the pterygoids from the quadrate to such an extent as to disguise their real nature and make them appear as additional ossifications. According to this view the pterygoid of Birds, the os transversum of Crocodiles, the transpalatine of the Snake, and the internal pterygoid of Mammals, including Man, arise in connection with the distal end of the palato-quadrate cartilage, and must therefore be regarded as homologous bones. The so-called pterygoids of Snakes and of Crocodiles and the basitemporals of Birds agree in their mode of development and relationship to the main morphological landmarks of the skull; they must therefore be regarded as homologous ossifications, and as a matter of convenience it is proposed to name them postpalatines. Whether these ossifications are represented in the mammalian skull by the so-called sphenoidal tubinais, or by certain accessory ossicles which are developed in connection with the hinder end of the vomer in some types (marsupials, hedgehog, &c), or not represented at all, is a matter of very little importance. It would of course be very interesting to be able to determine whether the bones which prolong the hard palate in some of the Edentates, Myrmecophaga for example, arise in the same manner as the bird's basitemporals. M y conviction, so far |
