No. 3-] THE VERTEBRATE HEAD. 505 



II. Historical Review of the Work on Neuromeres. 



The question of Metamerism of the Head as based upon 

 myotomes has been completely reviewed by Dohrn, Killian, and 

 others ; I shall say nothing on that side of the problem, but 

 shall limit the historical review, and confine the discussion to 

 the side of the question that has been less cultivated. 



It is a fact of comparatively recent discovery that the whole 

 neural tube of vertebrates is divided by constrictions into simi- 

 lar segments. Each segment is bounded, anteriorly and poste- 

 riorly, by transverse folds ; and the elevated area between them 

 constitutes the segment to which the name metamere is given. 

 These segments may be pictured to the mind as a series of 

 transverse ridges and furrows occupying each side of the 

 neural tube and not extending across the median plane. They 

 are exhibited in very young embryos of Vertebrates and disap- 

 pear before what may be called the middle embryonic period. 

 The existence of such folds in the walls of the hind-brain has 

 been known since the time of Von Baer, who in 1828, first 

 observed them in the embryonic chick of the third day of 

 development ; but it was not until 1889 that they were known 

 to extend throughout the length of the neural tube. 



Since Von Baer's time they have been observed and com- 

 mented upon by various anatomists. Bischoff ^ figures the 

 neural segments, but does not mention them either in the text 

 or in the descriptions of the figures. His figures show seven 

 folds in the region of the fourth ventricle of a dog embryo of 

 the twenty-fifth day of development. There are also shown 

 three additional folds in the region of the mid-brain. 



Remak, in 1850, made important observations, and suggested 

 that the segments in the hind-brain are connected with the 

 origin of the nerves in that region. He noted five or six quad- 

 rilateral fields on each side of the hind-brain walls, calling 

 attention to the fact that they correspond closely in position 



1 I am greatly indebted to Hoffmann's historical review of the literature in 

 Bronn's Klassen und Ordnungen des Thierreichs. I have consulted nearly all the 

 literature referred to there, but, in some few cases, where the original papers have 

 been inaccessible, I have depended wholly upon his review of it. 



