338 JOHN WARREN 



brain segments. Neal says his first three vesicles correspond to 

 Zimmermann's fore, mid and hind brain. However, Zimmerman's 

 seventh and eighth hind brain segments were not at first developed 

 and so Neal concludes that there are six primary 'encephalomeres/ 

 not eight. The divisions of the primary fore brain into 'Secun- 

 dare Vorderhirn' and ' Zwischenhirn ' Neal does not consider 

 as morphologically equivalent to neuromeres because as Herrick 

 stated they are really dorsal expansions and should not be com- 

 pared with ventral expansions. Neal agrees with Zimmermann that 

 the primary mid brain divides into three segments of which the 

 anterior lies in front of the posterior commissure. But accord- 

 ing to my figures this should go with the fore brain to form its 

 synencephalic segment (Kupffer). Neal concludes that struc- 

 tures of different morphological value have been described as 

 neuromeres in the brain in front of the cerebellum. These struc- 

 tures are really secondary subdivisions which differ from typical 

 neuromeres in shape, size, time of appearance and relation to dorsal 

 and ventral zones. He thinks it best to regard each of the pri- 

 mary fore brain and mid brain vesicles, neuromeres I and II, as 

 being serially homologous with the hind brain neuromeres III-IV. 

 He feels however that on ''basis of structure and relation to other 

 segmentally arranged organs that the primary vesicles, fore brain 

 and mid brain give evidence — as do the primary expansions of the 

 hind brain — ^of the primitive segmentation of the vertebrate head." 

 Hill (42, 43) worked under Locy on Salmo purpuratus studying 

 both dead and living specimens. He found like Locy eleven seg- 

 ments separated by grooves running around the whole brain. 

 Of these five were found in the mid brain and fore brain. These 

 segments he called the primary neuromeres and they antedated 

 the three fore, mid and hind brain vesicles. They all had the 

 typical characteristics described by Orr and were the same as 

 those in the medulla. Essentially the same results were seen in 

 young chicks. Three segments in the fore and two in the mid 

 brain could be seen on the open and later closed tube. Later 

 after the vesicles appeared the outlines of the primary neuromeres 

 disappeared in the anterior part of the brain first and secondary 

 subdivisions appeared in the fore brain, but Hill did not think 



