MUSEUM OF COMPARATIVE ZOOLOGY. 179 



In respect to the further development of these pro to vertebrae, I quote 

 from Froriep, 1 the results of whose investigations have been corroborated 

 by Van Wijhe. "In der Occipital region, d. h. in dem zwischen ersten 

 Cervical-nerven und Vagus eingeschlossenen Abschnitt der Wirbelsaule, 

 finden sich bei viertagigen Huhnerembryonen vier Muskelplalten, welche 

 von hinten nach vorn (cranialwarts) an Grbsse abnehmen. Es waren 

 also hier vier Urwerbel angelegt, welche in den Aufbau des Kopfes 

 eingehen. Der funfte liegt auf der Grenze von Kopf und Halswirbelsaule, 

 die Muskelplatte, die dieser liefert, beruhrt mit ihrem caudalen Rand 

 die Anlage des ersten Halswirbelbogens, mit dem cranialen die hinterste 

 Bogenanlage der Occipitalgegend." 



We see, therefore, that the first mesodermic cleft divides evenly the 

 four pairs of protovertebra? which enter into the formation of the head, 

 and that this cleft does not coincide with the occipito-cervical cleft of 

 the adult. 



B. — The Relation op the Cranial and the Spinal Nerves to 

 the Neural Axis. 



The development of the rudimentary protovertebra b brings me 

 to that stage in the growth of the chick with which my work ostensibly 

 began. Contemporaneously with the formation of this protovertebra, 

 the successive constrictions of the medulla appear, the neural crest is 

 formed, and the first cranial nerves arise (V., VII., VIII.). 



If a chick be examined by transmitted light during the second or 

 third days of incubation, it will be noticed that the medulla is marked 

 by a series of swellings and constrictions which are directly continuous 

 with a line of similar swellings and constrictions in the region of the 

 spinal cord. These divisions of the medulla are such noticeable char- 

 acteristics that they have frequently been mentioned before the com- 

 paratively recent attempts to determine the number of head-segments 

 from the distribution of the cranial nerves. His, Rabl, and Balfour 

 speak of them as more or less transitory structures ; but in so far as I 

 know, Beraneck and Orr are the only authors who have attempted to 

 elucidate by their means the problem of cranial segmentation. 



In an article upon the cranial nerves of the Lizard, 2 Beraneck 

 describes five successive enlargements of the central canal of the 



1 Archiv Anatomie und Physiologie, 1883. Zur Entwickelungsgeschichte der 

 Wirbelsaule, p. 226. 



2 Des Nerfs Craniens chez les Le'zards. Recueil zoologique Suisse, 1884 v 



