590 DB. W. G. aiDBWOOD ON TUB HYOBRANCIIIAL [May 18, 



evidently to be regarded as the remnant of the external boundary 

 of the thyroid foramen. 



Probably the most important fact from a morphological point of 

 view to be learned from this stage is the mode of formation of the 

 postero-lateral processes of the hyobranchial skeleton oF the adult 

 (ppl., fig. 9). The proximal portion of the first ceratobrauchial 

 does not share in the general absorption of ceratobranchial and 

 commissural cartilage, but is continued as a tapering central core 

 into the middle of the distal part of the arch, which is disinte- 

 grating. This central core is of firm cartilage similar to that of 

 the hypobrauchial plate and has a sharp contour. The postero- 

 lateral process of the adult is formed by the subsequent enlarge- 

 ment of this persistent proximal portion of the first ceratobranchial, 

 and the sinus which in the later stages exists between the postero- 

 lateral process and the thyrohyal lies morphologically between the 

 cartilage bounding the second branchial cleft antero-externally and 

 the cartilage which forms the postero-internal border of the thyroid 

 foramen. Saint-Ange (17. p. 418, Rana), Duges (4. pp. 98-99, 

 pi. 13. figs. 76-79, Pelohates), and Gdtte (6. figs. 332-334, 

 Bomhinator) are all agreed that the process in question is either 

 the persistent basal portion of the first branchial arch or else an 

 outgrowth of that part of the hypobrauchial plate with which the 

 first branchial arch was formerly united \ 



On the under surface of the hyobranchial skeleton the ventral 

 splint-bone has appeared, but it is not yet adherent to the 

 cartilage. It is a single bone from the first, and is not formed by 

 the coalescence of a pair. The mandible is now shaped as in the 

 adult, and except in size and the extent of ossification does not 

 differ materially from that of the latter. 



Stage 7. Distance from snout to cloaca, 21 mm. No tail. 

 Length of hind limb, extended, 30 mm. Lenr/th of fore limb, 

 extended, 12 mm. (Plate XXXV. fig. 7.) 



Those parts of the branchial skeleton which were undergoing 

 absorption in the preceding stage have now completely disappeared, 

 so that the thyrohyal is separated from the postero-lateral process 

 by a deep sinus as in the adult. The basal plate or body of the 

 hyoid is now a continuous sheet of cartilage in which the outlines 

 of the five constituent parts can no longer be distinguished, 

 except that there still exists near the middle the Y'shaped mark 

 adverted to in the description of Stage 6. AVhether the hyoglossal 

 notch deepens by absorption of the cartilage of the basal plate as, 

 according to Gaupp (5), occurs in Rana fusca, I am unable 

 definitely to say. There is no trace of the soft cai-tilage which is 



^ Since Gaupp (5. p. 428) has already exposed the fallacy of Parker's views 

 on tlie mode ol origin of the aatero-lateral and postero-lateral processes of the 

 Frog's hyobranchial skeleton, and has called attention to the confusion which 

 Walter (21. p. 6) ha.s introduced into the subject by calling the antero-lateral 

 process the " hypoliyale," — the result of a mistaken reading of Parker's text- 

 book (13. p. 173), — I refrain from further criticism of these works. 



