SKELETOIS" AND LARYNX OF XENOPUS AND PIPA. 



105 



takes place at the free extremities of the cornua, there is no 

 diminution in the total width of the hyoid skeleton. The most 

 important change which has taken place in the branchial 

 skeleton is the severing of the commissural or epibranchial 

 cartilage between the lateral extremities of the first and second 

 branchial arches. An indication of this is to be seen in Parker's 

 figure (33. PI. 63. fig. 4) of a stage much earlier than this. It is 

 interesting as being the first step in the total disintegration of 

 the branchial arches which so soon supervenes. The larynx has 

 advanced in position so that it is now in contact with the 

 branchial skeleton. 



Stage III. (PI. 11. fig. 6.) 



It is impossible to give a diagnosis of embryos of this and the 

 two following stages, as a glance at the table of measurements 

 will show. The changes in the hyobranchial skeleton at this 

 period appear to be unaccompanied by any marked alteration in 

 the external appearance of the embryo. 



The general aspect of the hyobranchial skeleton is now entirely 

 changed, and the absorption of cartilage is proceeding rapidly. 

 The hyoid cornua are reduced to rods of cartilage, while tlie 

 branchial arches are breaking up, chiefly at their peripheral ends. 

 Before proceeding to a detailed description of the skeleton at 

 this stage, it may be well to point out that in Xenopus the hyoid 

 arch remains practically unchanged during the time that the 

 branchial arches are undergoing their gradual reduction, but that 

 in the early stages of Pipa it becomes rapidly diminished and the 

 branchial skeleton follows but slowly. At Stages III. and lY. 

 of Pipa the raj)id absorption of the branchial arches begins, and 

 then it is a race, as it were, between the hyoid and branchial 

 arches for first disappearance. The rate of absorption at this 

 period is excessive, so much so that embryos the hyobranchial 

 skeleton of which has reached Stages III. and IV., and even V., 

 are externally indistinguishable : in other words, that the rate 

 of change in the hyobranchial apparatus exceeds by a good deal 

 that of the general metamorphic changes of the body. 



In this Stage III. the diameter across the branchial skeleton 

 is not greater than that across the hyoid arch. This may be 

 partly due to an elongation of the hyoid cornua; but the more 

 important factor in this change of proportion is undoubtedly a 

 shriiiTcage of the branchial arches. This is indicated to some 



