PARAPHYSIS AND PINEAL REGION IN REPTILIA 333 



DISCUSSION 



Subdivision of the fore brain 



The question of segmentation of the neural tube has been one 

 of great interest for many investigators and much has been 

 written on the segmentation both of the medullary groove and 

 of the neural tube after its closure. The earlier writers confined 

 their attention chiefly to the hind brain and the medulla where 

 the neural segments appear to best advantage. Von Baer ('28) 

 observed folds on the lateral wall of the medulla in the chick. 

 Bischoff C45), Remak C50-'55), Dursy ('69), Dohrn ('75) saw 

 these folds in the medulla of the dog, chick, cow and bony 

 fish embryos. Beraneck ('84) studied those segments with refer- 

 ence to their relations to the cranial nerves in lacerta, and 

 he (7) and Prenant (83) in the chick also. Kupffer (57) found 

 five pairs of segments in the medulla and three in the mid 

 brain of trout embryos. He stated that he could then find no 

 segments in front of the mid brain. He found later in Salamandra 

 atra a regular segmentation of the wide open neural plate and 

 counted eight pairs of neuromeres. He showed therefore that 

 there was present at this early stage an ' ontogenetisch-primare 

 neuromerie' extending throughout the brain plate. He was 

 unable to state definitely how many segments belonged to the 

 fore and mid brain as the line of demarcation between these 

 regions was ill-defined at this early stage. 



Orr (77) studied the segmentation in the lizard on the closed 

 brain tube and applied the term neuromere to the ' ' replis medul- 

 laires" of Beraneck. In his earliest stages the three brain vesicles 

 fore, mid and hind brain were formed. He found that the optic 

 vesicles developed from the lateral wall of the extreme anterior 

 part of the primary fore brain so that the anterior walls of the 

 optic stalks were in the same plane with the anterior surface of 

 the fore brain. The hind brain neuromeres were well marked and 

 their histological structure quite characteristic. Each one was 

 separated from its neighbor by an internal ridge and an external 

 groove and each pair were placed exactly opposite each other. 

 The cells were elongated and placed radially to the inner curved 



