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extending across the median plane. They are exhibited in very young 

 embryos of vertebrates, and degenerate before what may be designated 

 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, but it was not until 1888 that they were 

 known to extend throughout the neural tube. 



Since von Baer's time they have been observed and commented 

 upon by various anatomists. Remak, in 1850, made important observ- 

 ations and suggested that the segments in the hind-brain were 

 connected with the origin of the nerves in that region. Dursy ob- 

 served them in 1869. Foster and Balfour, in 1874, again suggested 

 that they were of segmental importance. Dohrn accepted and ex- 

 tended this view in 1875. 



In 1877, as competent an authority as Mihalkovics was in- 

 clined to interpret these segments as due to mechanical pressure of 

 the mesoblast, and, therefore, not a fundamental feature of the 

 medullary tube, but this view has since given way to a better ac- 

 quaintance with the facts, and they are now generally conceded by 

 morphologists to be of segmental value, and to represent a primitive 

 segmentation of the neural axis. 



Beraneck, in 1884, showed that there is a definite connection 

 between certain of these segments and cranial nerves, thus giving, 

 for the first time, a real foundation for establishing their segmental 

 relations. Kupffer maintained, in 1886, that these segments indicate 

 a primary metamerism of the medullary tube. He found, also, the 

 folds, or segments, extending into the spinal cord region, but 

 could not trace them to its hinder end. 



Orr, in 1887, traced very definitely the connection between these 

 segments in the hind-brain and cranial nerves. In describing the 

 segments he made use of the term "neuromeres" which has since 

 been generally adopted on this side of the Atlantic. He described 

 six in the hind-brain of the lizard (Anolis) giving their anatomical 

 characteristics with great clearness. He observed no neuromeres be- 

 hind the point of origin of the 10th nerve, nor did he find them in 

 the fore- and mid-brain, but he concluded, hypothetically, that they 

 were present in the anterior brain regions. 



Hoffmann, in Bronn's Klassen und Ordnungen des Tier-Reichs, 

 1888, records his observations on these segments in Lacerta and 

 Tropidonotus. In these animals he found seven segments in the hind- 

 brain connected with cranial nerves. 



