PARAPHYSIS AND PINEAL REGION IN REPTILIA 345 



have had to touch on the mid brain slightly in this paper and the 

 drawings and models take in this region. I have not gone into 

 it deeply enough to express SiUj definite opinion though it seems 

 to me probably that there are two subdivisions in this region. 

 Where three have been mentioned it is possible that the most 

 cephalic segment really belongs to the diencephalon and is that 

 portion known as the 'Schalthirn' or synencephalon. As the 

 cephalic end of the posterior commissure develops here that might 

 explain the reason for adding it to the mid brain. 



The three subdivisions of the fore brain have made their appear- 

 ance after the formation of the fore brain vesicles. The question 

 now arises, Are these true neuromeres equivalent to those in the 

 hind brain, which appeared before the three primary central ves- 

 icles were developed, or are they merely secondary subdivisions 

 of the fore brain appearing later than true neuromeres and having 

 a different morphological value? Kupffer in Hertwig's Handbuch, 

 1903, pp. 19 and 152-166, gives a careful review of the work done 

 on neuromeres. He distinguished between primary neuromeres 

 seen on the open neural plate, which extend throughout its whole 

 length, and secondary neuromeres, which appear later in the 

 closed tube and are especially well marked in the hind brain, but 

 much less distinct in the fore and mid brain. The expression 

 primary is to be understood in an "ontogenetic not in a phyloge- 

 netic sense." These secondary neuromeres are better marked on 

 the roof and lateral walls than on the floor of the brain. 



Orr did not consider that the fore brain neuromeres were true 

 neuromeres like those in the hind brain as their structure did not 

 conform to that of hind brain neuromeres. McClure states that 

 these fore brain neuromeres are true as regards their structure 

 and character and he considers that they are continuous with and 

 equivalent to those in the hind brain, but he suggests that there 

 may be a rudimentary neuromere in the fore brain. Waters and 

 Weber give the fore brain neuromeres full metameric value and 

 Locy considers that his early segments can be traced into those 

 structures that later become neuromeres. Herrick feels however 

 that dorsal structures have been homologized with ventral and 

 that if fore brain neuromeres were ever present they would be 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 11, NO. 4 



