DEVELOPMENT OF SKULL: RATHKE 141 



by Carus, Treviranus, de Blainville and Geoffrey, to establish 

 by anatomical comparison the homologies of the opercular 

 bones, for he could show that these bones were peculiar 

 to fish, and were scarcely indicated, and that only temporarily, 

 in the development of other Vertebrates. 1 He did not, 

 however, himself realise the relation of the ear-ossicles to the 

 gill-arches, though he knew that Spix and Geoffrey were 

 quite wrong in homologising them with the opercular bones 

 in fish. He described, it is true, the development of the 

 external meatus of the ear and the Eustachian tube from the 

 slit which appears between the first and the second arch, as 

 Huschke had done before him ; he described, in confirmation 

 of Meckel, the " Meckelian process " of the hammer running 

 down inside the lower jaw ; but the discovery of the true 

 homologies of the ear-ossicles was not made until a year or 

 two later by Reichert. 



In his further study of the development of Blennius vivi- 

 parus, Rathke observed some important facts about the 

 development of the vertebral column and skull. He found 

 that the vertebral centra were first formed as rings in the 

 chorda-sheath, which give off neural and haemal processes. 

 The vertebra later ossifies from four centres. The chorda 

 (notochord) is prolonged some little way into the head, and 

 the base of the cranium is formed by the expanded sheath, 

 which reaches forward in front of the end of the notochord. 

 This cranial basis shows a division into three segments, 

 in which Rathke was inclined to see an indication of three 

 cranial vertebrae. (It turned out that this division into three 

 segments did not really exist, and Rathke later acknowledged 

 that he had made an error of observation.) The side walls of 

 the skull grow out from this base and form a fibrous capsule for 

 the brain. The cranial section of the chorda itself shows no 

 sign of segmentation ; but later on the cranial portion of 

 the chorda-sheath ossifies, like the vertebrae, from several 

 centres. The vomer, which, in the classical form of the 

 vertebral theory of the skull, was the centrum of the fourth, 

 or foremost, cranial vertebra, does not, according to Rathke, 

 develop in continuity with the cranial basis and the chorda 

 sheath, but develops separately in the facial region. 



1 Kiemenapparat) pp. 107-118. 



