II NEUROMERY 101 





pineal oi'Liiiii \vitli the right; (thougli also with tin- posterio 

 missure). Again in various Vertebrat.es (Teleostomatoufl lislies 

 Hill, 1894: Amphibians, Birds Cameron, 1903, 1904) the parapineal 

 nr^an is in early stages slightly to the left of the pineal oi-^.-m. 



On the whole it does not appear to the pivM-nt \\i-iti-r that the 

 evidence is sufficient to make the view probable. In the Lampreys 

 the connexion of the parapineal hody with only the left hahenular 

 ganglion appears, as indicated above, to be secondary: it is originally 

 connected with both right and left. Again, to turn to the Kept ilia. 

 the eye is in Sphenodon connected with the left habenular ganglion 

 and in the Lacertilia with the right, although it seems perfectly 

 clear from the figures given by Dendy and Novikoff respectively 

 that the eye is morphologically the same organ in the two types 

 mentioned. Were it not so we should lie driven to the position that 

 of a, pair of pineal eyes originally present one has disappeared entirety 

 in Sphenodon and the other has disappeared equally completely in 

 the lizards. The improbability from a physiological point of view of 

 this having happened need not be accentuated. 



Here again, then, there seems to be up to the present no sufficient 

 reason for departing from the view that pineal and parapineal organs 

 were primitively median in position, one in front of the other. As 

 to their original significance we have no obvious clue : the absence of 

 convincing evidence that they were originally eyes does not of course 

 preclude the possibility of their having been originally some kind of 

 sense organ. 



NEUROMERY. It has been noticed in various Vertebrates, particu- 

 larly Elasmobranchs, Amphibians and Birds, that the neural rudi- 

 ment while still in the form of an open plate is sometimes divided up 

 by numerous and regular transverse markings. Whether this appear- 

 ance of segmentation is caused simply by the active growth in length 

 of the medullary plate or whether on the other hand it has some 

 deeper significance has not been conclusively determined. The name 

 neuromere has been given to the apparent segments. That these 

 are really primitive morphological segments as is believed by many 

 and as is implied in the termination "-mere" seems improbable. 



The existence of a clearly marked segmentation of the nervous 

 system where it occurs in the phyla Annelida and Arthropoda is 

 brought about by the concentration of ganglion-cells in serially 

 repeated masses, in correlation with the presence of serially 

 repeated appendages (parapodia in Polychaeta), and there is no 

 sufficient evidence to show that such were ever present in the 

 ancestral Vertebrate. The fact that the longitudinal muscles are 

 divided into myotomes would not be sufficient by itself to account for 

 the external form of the central nervous system being segmented, for 

 in that case the segmentation would be still clearly marked in the 

 many tishes where the myotomes remain practically unmodified. 



During later stages, after the neural tube has become closed in, 

 " neuromeres" are particularly conspicuous in the brain region. 



