DEVELOPMENT OF THE SKULL IN THE CEOCODILIA. 265 



tube. The only permanent " cleft," the first of the series, scarcely opens externally, 

 and serves merely as the rudiment of the tympanic labyrinth of the adult ; it never 

 takes on any respiratory function. 



Thus the cranium, proper, and the visceral arches are greatly modified from what we 

 see in such a good fundamental type as the Skate, the cranium being " cribbed, and 

 cabin'd, and confined " within the fast-growing masses of the outer bones, whilst the 

 arches are arrested early, and then wrought into fitness for nevf physiological uses. 



Yet masked and curtailed as the elements of the skull are in so high a vertebrate 

 type, they are not too modified nor too fragmentary for interpretation ; the parts can, 

 besides their relative and functional names, have a terminology given them that shall 

 perfectly correspond witla that which has been given to those of the pure chondro- 

 cranium of the " Elasmobranchs," with its complex lattice-work of branchial arches. 



Thus, in the present paper, whilst I shall try to name every part by its own proper 

 morphological designation, I shall not disturb the old anthrqpotomical terms with which 

 the anatomist is familiar. 



I need scarcely say that all that is purely morphological in the present paper is based 

 on embryological facts ; and that whilst I would wish to make clear to the student 

 fresh from his human anatomy the structure and fitness of a skull so different from 

 that of Man, yet, on the other hand, the whole work of interpretation must be done 

 so as to commend itself to the mind of the embryologist. 



As the Crocodile is known to be one of the most ancient types inhabiting this 

 terraqueous globe, his development is full of interest in relation to those countless 

 Reptilian forms that have succumbed to secular changes of the earth, and have " left 

 neither son nor nephew " in tlie regions where they once were dominant. 



In the great structural conformity of the skull, in its early stages largely, and to a 

 wonderful degree also permanently, to that of the Bird, there is much both to admire 

 and to stimulate inquiry. The copious development of air-cells in the tympanic lahyrinth 

 is extremely like what we see in a Hornbill or a Toucan ; but the " final purpose " of 

 such a conformity is a dark riddle. On the other hand, fresh from the study of the 

 development of Lepidosteus, I find no difficulty in imagining a " Ganoid " descent for 

 the Crocodile. " I do read some tokens " of the gigantic forms of that older group 

 " in the large composition of this " Reptile. 



The skeleton of the head is composed of all the three embryonic layers — epihlast, 

 mesoMast, and hypoblast — only in its hinder half; the front part, from the pituitary 

 body forwards, is devoid of the lower layer. 



Therefore, in any comparison of the head with the trunk, this must be borne in 

 mind ; the notochord only reaches to the " infundibulum," which receives the pituitary 

 graft, and the hypoblastic lining of the throat ceases inside the mandibular arch ; 

 thence, above and below, the whole structure is composed merely of the upper and 

 middle layers. 



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