1G4 THE EMBRYOLOGICAL CRITERION 



Jacobson in 1842 1 introduced the useful term, " primordial 

 cranium," for the primitive cartilaginous foundation of the 

 skull, and drew a sharp distinction between cartilage bones 

 and membrane bones. 



In his RccJierches snr les Poissons fossiles* L. Agassiz used 

 Vogt's work on the development of Coregonus to establish a 

 classification of the bones of the skull in fish, a classification 

 which had the merit of drawing a sharp distinction between 

 the cartilaginous groundwork and the " protective plates " of 

 the fish's skull. He recognised that the protective plates 

 developed in a different way from the other bones of the 

 skull. " We must distinguish," he writes, " two kinds of 

 ossification ; one which tends to transform the primitive 

 parts of the embryonic cranium directly into bone, and 

 another which leads to the deposition of protective plates 

 round this core, which develop not only upon the upper 

 surface, as has hitherto been supposed, but also on the 

 lateral walls and on the lower surface of the cranium " (p. 112). 

 In the skull of all fish there are three elements (i) the 

 cartilaginous base, including the nuchal plate, the trabeculae 

 and the facial plate, together with the auditory capsules ; (2) 

 the cartilaginous cerebral envelope ; (3) the bony protective 

 plates (absent in Elasmobranchs). The bones developed in 

 relation to these cranial elements can be classified as follows : 

 (i) the basioccipital, exoccipitals (paroccipitals ? ), supra- 

 occipital and " petrous " (rocher), developed from the nuchal 

 plate ; the ali- and orbito-sphenoids developed from the 

 trabecula: ; the "cranial ethmoid " 3 developed from the facial 

 plate ; (2) the parietals, frontals and nasals formed from the 

 " superior " protective plate ; the " anterior " and " posterior " 

 frontals and the temporal, from the " lateral " plates ; the 

 body of the sphenoid and the vomcr from the "inferior" 

 plates. The other element, the cartilaginous brain-box, docs 

 not ossify, and tends to become absorbed (p. 124). 



In 1849 Kollikcr published a paper 1 dealing with the 



1 " Om Primordial-Cranict," ForhancUingar Skand. Naturf. Jlfi>/e, 

 Stockholm, 1842. 



- Vol. I., General part, pub. 1844. 



Entosphenoid, Owen. 

 1 Ziucitcr Bericht zootom. Anstalt zu \\'itrzburg, 



