Till: EMBRYOLOGICAL CRITERION 



skull in Annnocoetes, which in this respect is the simplest of 

 all Vertebrates (pp. 6, 22). In Aininococtcs, as Johannes 

 Mtiller had shown, the foundation of the skull is formed by 

 two long cartilaginous bars, between the hinder portions 

 of which the notochord ends. In these Rathke was inclined 

 to see the homologues of his trabecuku, and of the para- 

 chordals which he was ready to assume from his embryo- 

 logical observations. 



Miiller was, of course, very ready to accept Rathke's 

 opinions on this subject, for he considered that they 

 supported his own theory of the vertebral nature of the skull. 

 After describing in his HatidbncJi der PJiysiologic the cartila- 

 ginous bands in Ammocoetes and their highly differentiated 

 homologues in the Myxinoids, he writes in the later 

 editions, " Hence we see that in the cranium, as in the spinal 

 column, there are at first developed at the sides of the 

 chorda dorsalis two symmetrical elements, which subsequently 

 coalesce, and may wholly enclose the chorda. Rathke has 

 recently observed, in the embryos of serpents and other 

 animals, before the formation of the proper cranial vertebras, 

 two symmetrical bands of cartilage, similar to those which I 

 discovered as a persistent structure in Ainniococtcs. . . . 

 At a later period the basis cranii of vertebrate animals 

 contains three parts analogous to the bodies of vertebrae, 

 the most anterior of which, in the majority of animals, 

 is generally small, and its development frequently abortive, 

 whilst in man and mammiferous animals the three are very 

 distinct. These parts are developed by the formation of 

 three distinct points of ossification, one behind the other, in 

 the basilar cartilage." l 



Rathke was very cautious about accepting the vertebral 

 theory of the skull ; he saw that the facts of development 

 were not altogether favourable to the theory, and he gave his 

 adherence with many reservations and saving clauses. His 

 general attitude may be summed up as follows. - 



1 Ilandbuch der Physiologic dcs Afcnsc/tcn, Koblenz, 1835 ; Eng. 

 trans, by W. lialy, ii., p. 1615, 1838. 



; For a full staiemcnt of Rathke ; s conclusions," see the translation 

 given by Huxley in Lectures on the Elements of Comparative Anatomy, 

 London, 1864. 



