1898.] HYOtD or ALYTES OBSTEtEICANS. 9 



is caused partly by the now indistinct lateral limits of the second 

 copula, but mainly by the white fibrous tissue, from which the 

 ventral splint-bone will later develop, showing through the thick- 

 ness of the cartilage. 



The absorption of cartilage in the hypobranchial plates has 

 proceeded apace, and the thyroid foramina are now quite large 

 crescentic spaces. One of the consequences of this absorption is 

 that the lateral promontory of the hypobranchial plate to which 

 the first ceratobranchial is attached now stands out boldly at 

 right angles to the median plane. On the anterior edge of this 

 process a new cartilage is developing. It is as yet distinct from 

 the hypobranchial cartilage, but in Stage 7 it fuses on, and forms 

 part at least of, the alary or antero-lateral process of the adult 

 hyoid (see figs. 4-7). A pair of cartilages similarly placed are 

 figiu-ed by Parker in the hyobranchial skeleton of a recently 

 metamorphosed specimen of Rana palustris (7. pi. 5, fig. 9), but 

 he speaks of them (p. 37) as " remains of the branchial pouches," 

 a determination which their position shows to be untenable. 



The absorption of hypobranchial cartilage by the enlargement of 

 the thyroid foramina causes the ceratobrauehials to be drawn in, 

 so that the proximal ends of ceratobrauehials 2 and 3, which in 

 Stage 3 were in a line \^ith the extremity of the thyrohyal and the 

 proximal end of the first ceratobranchial, are now much closer to 

 the middle line. This, of course, is partly to be accounted for by 

 the growth in length of the thyrohyals. The general result is 

 that a transverse line drawn through the posterior ends of the 

 thyrohyals now passes behind the branchial skeleton, whereas in 

 the preceding stage the line passed through it. The fourth 

 ceratobranchial has almost disappeared, only its distal end, fused 

 with the external edge of the posterior extremity of the thyrohyal, 

 remaining. 



Stage 5. Spechrun measuring 17 mm. from snout to cloaca. 

 Stump of tail 1 mm. Length of hind limh, extended, 23 mm. Length 

 of fore limb, extended, 10 7nm. (Plate II, fig. 5.) 



The ceratohyals are slightly longer than before and considerably 

 thinner. The distal or posterior end is curved, and the part which 

 forms the lateral boundary of the hyoglossal sinus is quite slender. 

 The hyoglossal sinus itself is both broader and deeper. In Stage 4 

 the ceratohyals were in contact in the middle line, but they are 

 now considerably separated, and the extent of their divarication is 

 marked by two slight notches at the bottom of the hyoglossal sinus. 

 There is evidently an absorption of cartilage taking place here, 

 which causes the posterior copula to extend into the sinus. The 

 side margins of the postei'ior copula are still to be seen, but since 

 the fibrous predecessor of the ventral splint-bone underlies them, 

 it is only possible to obtain convincing proof of the fact after 

 removal of this superficial tissue. The lines run down to the antero- 

 intemal border of the thyroid foramen, and reach the hyoglossal 

 sinus in front at the notches already indicated. 



