144 THE EMBRYOLOGICAL CRITERION 



separate pieces. M tiller could not prove this point from the 

 available embryological data, and indeed the facts which he 

 did use had to be twisted to suit his theory. A curious 

 apparent confirmation of his idea that the centra of the 

 cranial vertebrae are formed from separate halves was 

 supplied in 1839 by Rathke's discovery of the trabeculas in 

 the embryonic skull of the adder. 



The next big step in the study of the development of the 

 skull was taken by a pupil of Miiller, C. B. Reichert, who 

 showed in his work very distinct traces of his master's 

 influence. Reichert's first, and most important contribution 

 to the subject was his paper on the metamorphosis of the 

 gill, or, as he called them, the visceral arches in Vertebrates, 1 

 particularly in the two higher classes. Reichert describes 

 the similar origin in embryo of bird and mammal (pig) of 

 three " visceral " arches. These arches stand in close relation 

 to the three cranial vertebrae which Reichert, like Miiller, 

 distinguishes. He makes the retrograde step of admitting 

 only three aortic arches, and he is not inclined to consider 

 the three visceral arches as equivalent to the gill-arches of 

 fish in his opinion they have more analogy with ribs, though 

 differing somewhat from ribs in their later modifications. 

 The visceral arches are processes of the visceral plates 

 (von Baer), which grow downwards and meet in the middle 

 line, leaving between one another and the undivided body 

 wall three visceral slits opening into the pharynx. The 

 first visceral process is different in shape from the others, 

 for it sends forward, parallel with the head and at right 

 angles to its downward portion, an upper portion in which 

 later the upper jaw is formed. The other two processes 

 are straight. From the hinder edge of the second visceral 

 arch there develops, as Rathke had seen, a fold which 

 is comparable with the operculum of fish. The first slit 

 develops externally into the car-passage, internally into the 

 Kustachian tube, and in the middle a partition forms the 

 tympanic ring and tympanum. Inside each of the visceral 

 processes on cither side a cartilaginous rod develops. In 



1 "I'eber die Visccralbo^en clcr \Yirbelthicre in All^cmeinen und 

 dercn Mctamorphoscn bci den V<i-<-ln und Siiugetliiere," Miiller's 

 Arclih 1 , |>|). 120-2-22, 1837. 



