THE SKELETON OF THE VERTEBRATE HEAD. 93 



changes undergone by them in development, prove these 

 parts to be serially homologous with the " visceral laminae," 

 and to be, like them, indications of the segmented structure of 

 the head in front of the so-called first visceral arch. The so- 

 called superior maxillary lobes indicate a segment of which 

 the palate and pterygoid bones are elements. The " lateral 

 frontal" indicate a second segment, containing the maxillary, 

 malar, and lachrymal bones. The external margins and angles 

 of the "anterior or frontal processes" indicate an inter- 

 maxillary segment ; and the development of the mesial part 

 of the same process into the cartilages of the nose indicates a 

 segment probably only fully developed in the mammalian 

 head. 



In addition, therefore, to the " visceral laminae" behind 

 that one in which the mandibular arch is formed, there would 

 appear to be a series of less developed " visceral laminae" in 

 front of it, all of which, in addition to other structures, give 

 rise to haemal arches of the sclerome, and indicate a number 

 of corresponding sclerotomes. 



Of the Primary or Fibrous Sclerome. The bones and 

 cartilages to which, from their palpable character, the atten- 

 tion of anatomists has been hitherto chiefly directed, are parts 

 only of 'the vertebrate sclerome. They are imbedded in a 

 continuous fibrous matrix, which, variously modified, binds 

 them together, and co-operates in their general economy and 

 functions. This matrix forms a more extensive, and, in some 

 respects, a more important element of the sclerome in the 

 lower than in 'the higher vertebrata ; and if viewed in the 

 former in connection with its early stages of development in 

 the embryo, it will be found to be arranged on the plan of the 

 "primordial vertebral system." It is most satisfactorily 

 studied in the fish, and articularly in those forms in which 

 the bones and cartilage are feebly developed. The fibrous ele- 

 ment of the sclerome forms the sheath of the " corda dorsalis" 



