No. 3-] THE VERTEBRATE HEAD. 509 



concavity between two segments, and not from the crest of a 

 neuromere. My observations on the motor roots agree with 

 those of Miss Piatt in that particular. 



Zimmerman ('91) states that he finds in embryos of the rab- 

 bit and chick, shortly before the closure of the neural groove, 

 the segments observed by Kupffer in Salamandra atra. He 

 noticed at first eight of these segments (encephalomeres) in the 

 brain region. The three anterior ones were much larger than 

 the five lying behind them in the medulla. The three front 

 ones are the vesicles of the fore-, mid-, and hind-brains, and 

 they straightway undergo secondary division as follows : The 

 first divides into two, the second into three, and the third into 

 three, making a total of eight secondary divisions arising from 

 three primary ones. These added to the five of the medulla 

 give a total of thirteen segments in the brain region. He also 

 observed these structures in Acanthias and Mustelus, and found 

 them very clearly defined. In mammals the metameres of the 

 mid-brain are not so distinct. Zimmerman goes on to say that 

 these folds cannot be accidental appearances, since in all classes 

 of vertebrates corresponding nerves arise from corresponding 

 segments. He gives a table, showing nerve relations, with too 

 much detail to reproduce here. 



Waters, whose complete paper appeared in June, 1892, 

 studied especially the mid-brain of Teleosts. He confirmed and 

 extended the observations of Orr and McClure. He counted 

 eleven neuromeres in the entire brain region : six in the hind- 

 brain, two in the mid-brain, and three in the fore-brain. He 

 did not find neuromeres in the brain of the Cod earlier than the 

 ninth day of development. He assigns the olfactory and 

 optic nerves to the anterior two neuromeres. The two neuro- 

 meres of the mid-brain give origin to the third and fourth 

 nerves, and from the six segments in the hind-brain the fifth, 

 seventh, eighth, ninth, and tenth nerves arise as designated by 

 McClure. The sixth nerve he found to occupy its theoretical 

 position when the neuromere exists ; when fusion has taken 

 place between the trigeminus and abducens neuromeres the 

 sixth nerve has been shifted backwards between the seventh 

 and eighth nerves. 



