Developmental history of primary segments of the vertebrate head. 401 



existed iu the fore-brain they would only be visible at an early stage 

 and would be obscured by altered conditions". 



According to Zimmermann, '91, there are 8 encephalomeres in 

 Salamander embryos iu front of the first protovertebra. The three 

 cephalad encephalomeres are large and subsequently divide, the first 

 into two and the second and third each into three, making a total of 

 13 cephalic segments. The author thinks the three large cephalad 

 encephalomeres are originally compound and that for some reason the 

 subdivision is retarded in this region. 



Finally, Locy, '95, traced the segments of the neural tube in 

 sharks back to very early embryonic stages before the eyes evaginate 

 or the somites of the body appear and claimed that they are identical 

 with the neuromeres of Orr and McClure. The results of my studies 

 on chick embryos tend to confirm the very early appearance of the 

 neural segments and to show that they are identical with the neuro- 

 meres so widely observed in later stages. Locy finds 11 segments 

 in front of the origin of the vagus nerve , 6 in the hind-brain , 2 

 in the mid-brain and 3 in the primary fore-brain. Later in the 

 ontogeny 3 segments from the spinal cord are incorporated in the 

 cephalic region, making a total of 14 cephalic segments. Neal, '98, 

 working upon the same material, finds the segments described by 

 Locy in the early embryonic stages but claims for them great 

 irregularity, and, failing to find them in embryos with closing neural 

 groove concludes that the early segments are mechanical formations 

 and in no way related to the neural segments that are present later 

 in the ontogeny. With Van Wijhe he finds 9 cephalic mesomeres 

 and also describes 7 encephalomeres, distributed as follows: the 

 fore-brain one, the mid-brain one, and the hind-brain five. 



Two important conclusions in reference to neuromeres appear 

 from the above review: 



1) There is substantial agreement among observers as to the 6 

 segments in the hind-brain, their nerve relation and metameric value. 

 When more than 6 have been observed (Hoffmann, '90, Locy, '95) 

 segments caudad to the true sixth have been counted. 



2) There are but few observations on the encephalic segments 

 that lie in front of the cerebellum. 



The following observations on the primitive segments of Teleost 

 and chick embryos were made during 1898 and 1899 in the Zoological 

 Laboratory of Northwestern University. It is a pleasure to acknow- 



