258 THE OLD MOUTH AND THE NEW. 



accomplished in the manner indicated for arthropods, and the successive steps in 

 the process may be followed with comparative ease in the frog. l In the study of 

 this process in the frog, large numbers of eggs were hardened, usually in chromic 

 or picric acid solutions, the membranes removed by eau de Javelle, and the most 

 sharply sculptured specimens examined as opaque objects, under a strong oblique 

 illumination. 



Soon after the closure of the medullary plate one may see the outlines of 

 two or three pairs of faintly marked ridges that represent the earliest stages of 

 the oral arches. (Fig. 159.) Behind them are two pairs of more prominent ones, 

 representing either the first two gill arches, or the hyoid arch and the first gill 

 arch. The primitive oral arches curve downward and forward, eventually unit- 

 ing between the anterior end of the brain and the anlage of the sucking disc, at 

 the point where the mouth appears later. (Fig. 160.) 



The general appearance of the primitive oral arches, and the rate at which 

 they concresce, varies considerably in different embryos, and it has not been pos- 

 sible to identify them with those that are seen in this region at a later period, 

 but the two shown in Figs. 159, 160, appear to represent the anlage of the pre- 

 maxillary and maxillary arches. After concrescence takes place, the arches, for 

 a short period, become indistinguishable. 



FIG. 159. Frog embryos, seen from side, showing the extension of the primitive oral arches, gill arches, and lateral 

 plates, toward the haemal surface. Rana septemtrionalis. 



In the following stages (Fig. 161, A), the oral region is bounded in front by a 

 conspicuous transverse groove that terminates at either end in the nasal pit, ol.o.} 

 on the posterior side it is bounded by the sucking disc. A longitudinal groove now 

 extends along the middle of the intervening space, intersected by three transverse 

 ones, a small pit being formed at each intersection. The oral field is thus divided 

 into at least three pairs of lobes that are clearly serially homologous with one 

 another. The first pair represents the premaxillae, p.mx., the second, the 

 maxillae, mx., and the third, the mandibles, md. 



In the following stages, the lobes become more prominent, and the longitud- 

 inal groove becomes deeper and wider, B and C. The premaxillary lobes then 



1 Mr E E. Just, 1906, and Mr. A. O. Kelly, 1908, students in biology at Dartmouth, have assisted in work- 

 ng out the history of the embryonic and larval jaws of the frog. 



