THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 629 



dissimilar, and more or less arbitrary, opinions upon this subject made 

 their appearance. An agreement even as to the number of vertebrse 

 contained in the skeleton of the head could not be reached. Some 

 investigators assumed six, others five or four, or even three only. 



HUXLEY, in his "Elements of Comparative Anatomy," by a critique 

 based upon facts, was the first to prepare the way for a termina- 

 tion of this unpleasant state of affairs, in which the vertebral 

 theory was held to with tenacity, notwithstanding the contradictions 

 that everywhere arose. In his discussion he argued from a series of 

 facts 'which embryological investigation had brought to light. As such 

 the following, important for the problem of the skull, should be 

 cited before all others. 



First, the discovery that the skeleton of the head, like the verte- 

 bral column, is developed out of a cartilaginous condition, and that 

 the brain is first enclosed by a primordial cartilaginous cranium 

 (BAER, DUGES, JACOBSON). 



Secondly, the doctrine established mainly by KOLLIKER, that the 

 bones of the head-skeleton are separable into two groups according 

 to their development into the primordial bones, which arise in the 

 primordial cranium itself, and the secondary or covering bones, 

 which have their origin in the enveloping connective tissue. 



Thirdly, the insight which was acquired, through the important 

 works of RATHKE and REICHERT, into the metamorphoses of the 

 visceral skeleton, and thereby into the development of the palato- 

 maxillary apparatus and the auditory ossicles. 



Through an examination of these various facts, HUXLEY was led to 

 the important and fully justified conclusion, that not a single cranial 

 bone can be recognised as a modification of a vertebra, that the skull 

 is no more a modified vertebral column than the vertebral column is a 

 modified skull ; that, rather, both are essentially distinct and different 

 modifications of one and the same structure. 



While HUXLEY stopped at the negative standpoint, simply denying 

 the vertebral theory, GEGENBAUR has made the question of the 

 relation of skull and vertebral column, raised by GOETHE and OKEN, 

 but from ignorance of the facts incorrectly answered by them, again 

 the object of profound comparative study. Rightly recognising 

 that the problem can be solved only by detailed investigation of 

 the primordial skeleton, he selects as the object for his studies the 

 cartilaginous skull of the Selachians, and endeavors in his revolu- 

 tionising work, " Das Kopfskelet der Selachier als Grundlage zur 

 Beurtheilung der Genese des Kopfskelets der Wirbelthiere," to 



