150 THE EMBRYOLOG1CAL CRITERION 



to their type " (p. 152, 1838). The skull of these lower fishes 

 is itself a specialised one; it is an individualised modification 

 of a simple type of skull. And this holds good in general of 

 the skulls of the lower Vertebrates they are individualised 

 exemplars of a simple general type, not merely unmodified 

 embryonic stages of the greatly differentiated skulls of the 

 higher Vertebrates (p. 250, 1838). Differentiation within the 

 vertebrate phylum is therefore not uniserial, but takes place in 

 several directions. Reichert describes two sorts of modifica- 

 tions of the typical skull class modifications and functional 

 modifications. The causes of the modifications which 

 characterise classificatory groups are unknown ; the second 

 class of modifications occur in response to adaptational 

 requirements. 



Reichert's t\vo papers are of considerable importance, 

 and Miiller's remark in his review 1 of them is on the whole 

 justified. "These praiseworthy investigations supply from 

 the realm of embryology new and welcome foundations for 

 comparative anatomy " (p. clxxxvii.). 



The development of the skull -was, however, more 

 thoroughly worked out by Rathke, and with less theoretical 

 bias, in his classical paper on the adder.' 2 This memoir of 

 Rathke's is an exhaustive one and d'eals with the develop- 

 ment of all the principal organ-systems, but particularly of 

 the skeletal and vascular. He confirmed in its essentials 

 Reichert's account of the metamorphoses of the first two 

 visceral arches, describing how the rudiment of the skeleton 

 of the first arch appears as a forked process of the cranial 

 basis, the upper prong developing into the palatine and ptery- 

 goid.the lower forming Meckel's cartilage, while the quadrate 

 develops from the angle of the fork. The actual bone of the 

 upper jaw (maxillary) develops outside and separate from 

 the palato-pterygoid bar. The cartilaginous rod supporting 

 the second visceral arch divides into three pieces on each 

 side, of which the lower two form the hyoid, the uppermost 

 the columella. Like Reichert he held the visceral arches to be 

 parts of the visceral plates, containing, however, elements from 

 all three germ-layers the serous, mucous, and vessel layers. 



1 Miiller's Archiv for 1838. 



- Entwickelungsgeschichte der Natter ) Konigsberg, 1839. 



