1871.] 



Skull of the Common Frog. 



247 



the osseous fish. I was working at the early conditions of the salmon's 

 skull at the time. 



I was, however, led to devote my attention to another and more instruc- 

 tive type early in the following year ; for it was then (January 1869) that 

 Professor Huxley was engaged in preparing his very important paper " On 

 the Representation of the Malleus and Incus in the other Vertebrata " (see 

 Zool. Proc. May 27, 1869). 



In repeating some of his observations for my own instruction, it occurred 

 to me to renew some researches I had been making from time to time on 

 the frog and toad. The results were so interesting to us both, that it was 

 agreed for me to work exhaustively at the development of the frog's skull 

 before finishing the paper on that of the salmon. On this account Pro- 

 fessor Huxley mentions in his paper {op. cit. p. 406) that he leaves the 

 Amphibia out of his demonstration, and that they are to be worked out 

 by me. The amount of metamorphosis demonstrable in the chick whilst 

 enclosed in the egg suggested a much more definite series of changes in 

 a low, slow-growing Amphibian type. I think that this has been fully 

 borne out by what is shown in the present paper. 



The first of the ten stages into which I have artificially divided my sub- 

 ject is the unhatched embryo, whilst its head and tail project only mode- 

 rately beyond the yelk-mass. Another stage is obtained by taking young 

 tadpoles on about the third day after they have escaped from their glairy 

 envelope ; a few days elapse between the second and third stages, but a much 

 longer time between the third and fourth, for the fourth stage is the perfect 

 tadpole, before the limbs appear and whilst it is essentially a fish with 

 mixed Chimceroid and Myxinoid characters. Then the metamorphosing 

 tadpole is followed until it is a complete and nimble frog, two stages of 

 which are examined ; and then old individuals are worked out, which give 

 the culminating characters of the highest type of Amphibian. 



The early stages were worked out principally from specimens hardened 

 in a solution of chromic acid ; and the rich umber-brown colour of these 

 preparations made them especially fit for examination by reflected light. 



Without going further into detail as to the mode of working my subject 

 out, and without any lengthened account of the results obtained, I may 

 state that the following conclusions have been arrived at ; namely, that the 

 skull of the adult is highly compound, being composed of : — 



1st. Its own proper membranous sac ; 



2nd. Of a posterior part which is a continuation, in an unsegmented 

 form, of the vertebral column ; 



3rd. Of laminae which grow upwards from the first pair of facial arches, 

 and which enclose the fore part of the membranous sac, just as the " in- 

 vesting mass " of the cranial part of the notochord invests the hinder part. 



4th. The ear-sacs and the olfactory labyrinth become inextricably com- 

 bined with the outer case of the brain. And 



5th. The subcutaneous tissue of the scalp becomes ossified in certain 



