Developmental history of primary segments of the vertebrate head. 399 



especially strengthened by showing (Locy, '93) that they antedate the 

 mesomeres even of the trunk region. On the whole there is better 

 agreement among the observers as to the number and character of 

 these segments than there is as to the mesoblastic divisions of the 

 head. The chief differences lie in reference to the fore- and mid- 

 brain regions. 



Early observations of folds in the hind-brain are recorded by 

 VON Baer, '28, who saw them in the chick during the third day of 

 incubation; by Bischoff who found them in the dog; by Remak, '50, 

 who suggested a possible nerve relation, and by Dursy, '69, who 

 counted 6 folds in the hind-brain of the embryonic cow. Dohrn, '75, 

 contrasted this segmentation in the medulla of bony fishes with the 

 metamerism of insects, thus suggesting a segmental value. Mihal- 

 Kovics, '77, and Balfour, '81, were inclined, however, to give a 

 mechanical interpretation to them. 



Although Remak in 1850 had suggested that the segments of the 

 medulla possibly bore a definite relation to the cranial nerves of that 

 region, this relationship was not determined or placed on a sub- 

 stantial basis till Béraneck in 1884 showed that, in embryos of 

 Lacerta agilis, from 3 to 7 ram in length, the connection of cranial 

 nerves with definite segments of the medulla was both constant and 

 regular. Béraneck thus contributed the first real factor in establishing 

 their segmental relations. In 1887 he reached similar conclusions from 

 observations on embryos of the chick. 



Kupffer's observations are extensive and cover a period of 

 several years. In 1884 he describes five segments in the medulla and 

 three in the mid-brain of trout embryos, 18 to 20 days old, assigning 

 to them a segmental value. He also finds 5 segments in the medulla 

 of Mammalia. In the following year '85, he observed in young embryos 

 of Salamandra atra with a wide open neural groove eight 

 cephalic segments. Each segment is bounded anteriorly and posteriorly 

 by transverse folds that occupy the median part of the cephalic 

 plate but does not effect the margins of the neural groove. In 

 1893 in his "'Vergleichende Entwicklungsgeschichte des Kopfes der 

 Cranioten" he reprints these conclusions. 



Orr, '87, describes in detail 6 segments in the hind-brain of 

 Anolis and traces very definitely their connection with the cranial 

 nerves of this region. He discusses the histological condition of these 

 segments ("neuromeres") and gives five criteria by which a typical 

 segment may be identified. (A discussion of these criteria ap- 



