424 



In the further course of development, the trabeculse approximate 

 and elongate, so as to obliterate the subpituitary membrane, and 

 form with the enlarged basal cartilage, the ethmoid cartilage and the 

 ethmovomerine cartilages, the continuous cartilaginous craniofacial 

 axis. A histological metamorphosis into cartilage is undergone by 

 the roof of the occipital region of the skull, but in front of this it 

 remains membranous ; so that in the adult frog (in which this car- 

 tilaginous framework persists), the skull, when deprived of its bony 

 matter, presents an anterior fontanelle. The ethmovomerine cartilages 

 diverge still more, and form the broad mass whose lateral cavities 

 shelter the olfactory sacs in the adult frog. If, bearing in mind the 

 changes which are undergone by the palatosuspensorial apparatus, 

 and which have already been described, we now compare the stages 

 of development of the frog's skull with the persistent conditions of 

 the skull in the Amphioxus, the Lamprey, and the Shark, we shall 

 discover the model and type of the latter in the former. The skull 

 of the Amphioxus presents a modification of that plan which is 

 exhibited by the frog's skull, when its walls are still" membranous 

 and the notochord is not as yet imbedded in cartilage. The skull 

 of the lamprey is readily reducible to the same plan of structure as 

 that which is exhibited by the tadpole, while its gills are still 

 external arid its blood colourless. And finally, the skull of the 

 shark is at once intelligible when we have studied the cranium in 

 further advanced larvae, or its cartilaginous basis in the adult frog. 



Thus, I conceive, the study of the mode in which the skulls of 

 vertebrate animals are developed, demonstrates the great truth which 

 is foreshadowed by a careful and comprehensive examination of the 

 gradations of form which they present in their adult state ; namely, 

 that they are all constructed upon one plan ; that they differ, indeed, 

 in the extent to which this plan is modified, but that all these 

 modifications are foreshadowed in the series of conditions through 

 which the skull of any one of the higher Vertebrata passes. 



But if these conclusions be correct, the first problem which I pro- 

 posed to you, Are all vertebrate skulls constructed upon a common 

 plan? is solved affirmatively. 



We have thus attained to a theory or general expression of the 

 laws of structure of the skull. All vertebrate skulls are originally 

 alike ; in all (save Amphioxus ?) the base of the primitive cranium 



