ARCHETYPE OF SKULL 147 



the quadrate, which formed a suspensorium for both arches ; 

 the lower part, as Meckel's cartilage, formed a foundation for 

 the bones of the lower jaw. Of arch II., the lower part 

 became the horn of the hyoid, the upper part had a varying 

 fate. In some Anura it formed the ossicle of the ear 

 (homologue of the columella of birds and the stapes 

 of mammals), in others it disappeared. In reptiles the 

 upper segment of the second arch formed, as in birds, the 

 columella. 



The account of the metamorphoses of the visceral arches 

 in Amphibia forms only a small part of Reichert's memoir of 

 1838, the chief object of which was to discover the general 

 " typus " of the vertebrate skull, and to follow out its modifica- 

 tions in the different classes. Von Baer had shown that the 

 generalised type appeared most clearly in the early embryo ; 

 Reichert therefore sought the archetype of the skull in the 

 developing embryo. He brought to his task the precon- 

 ceived notion that the skull could be reduced to an 

 assemblage of vertebrae, but he saw that comparative 

 anatomy alone could not effect this reduction ; he had 

 recourse, therefore, to embryology, hoping to find in the 

 simplified structure of the embryo clear indications of three 

 primitive cranial vertebras (p. 121, 1837). 



In the head he distinguished two tubes, the upper formed 

 by the dorsal plates, the lower by the ventral or visceral 

 plates. Both of these tubes were derived from the serous or 

 animal layer (cf. von Baer, snpra, p. nS). The walls of the 

 lower tube were formed by the visceral processes, within 

 which later the skeleton of the visceral arches developed. 

 The walls of the upper tube formed the bones and muscles of 

 the cranium proper. The facial part of the head was formed 

 by elements from both upper and lower tubes. The dorsal 

 tube showed signs of a division into" three cranial vertebrae 

 (Urwirbeln, primitive vertebrae). In mammals and birds, 

 as Reichert had shown in his 1837 paper, the three cranial 

 vertebras were indicated by transverse furrows on the ventral 

 surface of the still membranous skull (see Fig. 10, p. 148). 



Even in mammals and birds, however, the positions of 

 the eye, the ear-labyrinth, and the three visceral arches were 

 the safest guides to the delimitation of the cranial vertebrae 



