298 Royal Society : — 



monstration, 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- 

 gvowiug 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 

 subject is the unhatched embryo, whilst its head and tail project 

 only moderately 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 ob- 

 tained, 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 unseg- 

 mented 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 " investing mass " of the cranial part of the notochord invests 

 the hinder part. 



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

 combined with the outer case of the brain. And 



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

 tain definite patches ; these are the cranial roof-bones. Around the 

 mouth there are cartilages like those of the Lamprey and the 

 ChimcEra ; but these yield in interest to the proper facial bars, which 

 are as follows, namely : — 



First ])air, the " trabeculae." 



Second })air, the mandibular arch. 



Third pair, the hyoid arch. 



And fourth to seventh pairs : these are the branchials. 



These are all originally separate pairs of cartilaginous rods ; and 

 from these are developed all the complex structures of the mouth, 

 palate, face, and throat. The pterygo-palatine arcade is merely a 

 secondary connecting bar developed, after some time, between the 

 first and second arches. 



Meckel's cartilage arises as a segmentary bud from the lower part 



