38 OBSERVATIONS ON THE DEVELOPMENT OF RAIA BATIS. 



trils projects above the surface, and forms the upper lip, or the portion already spoken 

 of as having so strong a resemblance to the intermaxillary bud or protuberance of the 

 higher vertebrates (fig. 11, d). The edges of this form the inner borders of the nasal 

 grooves, the outer ones being the result of a corresponding thickening of the integu- 

 ments on the other side of this groove. Both inner and outer borders rise to the 

 same level. 



If now we compare the phases which these parts pass through in skates with the 

 permanent conditions of them in other Selachians, it will be found that, in one species 

 or another, these permanent conditions are arrests of development in various stages. In 

 Oxyrhina gomphodon the nostrils retain permanently the primitive form of the olfactory 

 fossae, like that of the youngest skates noticed above ; in Pristiurus melanostoma, and in 

 Mustellus, Carcharias laticaudus, and many other species, the nostrils have superadded 

 the lobe on their inner border, but no further thickening of the integument between 

 them ; in Scyllium Bergeri this lobe is extended by the thickening of the integument 



towards the middle line, as in Fig. 10; and in Raia torpedo, Scyllium maculatum, 

 Taniurus Meyeri, and others, it extends across the whole space between the nostrils, 

 and forms above the mouth a continuous upper lip, as already described. 



If the part the development of which has just been described is to be compared with 

 the intermaxillary bud, the grooves on either side must be compared with the unclosed 

 nostrils of the embryos of air-breathing vertebrates, or in other words to the " hare-lip." 

 In air-breathing animals the nostrils open into the mouth, either by a canal between the 

 maxillary and intermaxillary bones, as in many reptiles, or by a canal extending farther 

 back, and separated from the mouth by the bones just mentioned and the palatines 

 in addition, as in mammals ; or by these bones and the pterygoids, as in the crocodiles. 

 In the Proteus, Axolotl, and Menobranchus, however, the nostrils cannot be said to 

 enter the mouth at all, but pass through the upper lip at a point corresponding with 

 the union of the maxillaries and intermaxillaries, but still exteriorly to the dental arch. 

 Bearing this in mind, we are led to look for the homologues of these bones in the 

 immediate neighborhood of the nostrils in the skate. The only parts which occupy 

 the position indicated are the cartilages, already referred to, contained in the nasal 

 lobes, and in the parts just outside of the nasal groove. Is not the cartilage which 

 extends from the olfactory fossae towards the pectoral fin the homologue of a maxillary 

 bone, and that in the lobe, of an intermaxillary * If so, the skates and Proteifonn 

 reptiles agree in having the nostrils open in front of the dental arch, but at a point 

 corresponding with the union of the maxillaries and intermaxillaries ; they differ in 

 this, that while in all Batrachians the nasal groove becomes closed, in the skates it 



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