134 BIOLOGICAL LECTURES. 



The history of these segments, in Acanthias and the chick, 

 has been followed very carefully, and the earliest formed ones 

 have been traced without a break into later stages and identi- 

 fied with the neuromeres. If, therefore, the segments of the 

 open neural groove stage are artifacts, it may with equal force 

 be claimed that the so-called neuromeres, which are their later 

 stages, are also artificially produced. 



It should also be borne in mind that similar segments exist 

 in correspondingly early stages in Amblystoma, Rana palustris, 

 the newt, and the chick, which indicates that they are not con- 

 fined to isolated cases but are a fundamental feature of verte- 

 brate development. 



The most satisfactory indication of their true nature is found 

 by observing living material before it has been brought into 

 contact with any reagent. Fortunately, the chick offers at 

 all times a source where we can get living embryonic material 

 of any desired age. These segments have been repeatedly 

 observed in living chick embryos of the eighteenth to twenty- 

 second hour of incubation, and have been treated with re- 

 agents while they were actually under observation. The effect 

 of the addition of picro-sulphuric acid is to render, immediately, 

 the walls of the neural groove opaque and more clearly defined, 

 but not to affect the number or arrangement of the segments. 

 The same segments have also been studied in living embryos 

 of Amblystoma. These facts are conclusive ; if the segments 

 exist in living embryos, they are veritable anatomical structures. 



Two points of fundamental importance may now be regarded 

 as established : (i) that the neural segments are present in 

 extremely early stages of vertebrates where they have not 

 heretofore been recognized, and (2) that they are true anatomi- 

 cal structures and not artifacts. The question still remains, 

 Do they furnish the best or even a good clue to the number of 

 segments in the primitive brain ? If so, they must be shown 

 to be equally important in this direction with myotomes, bran- 

 chiae, and cranial nerves. 



In estimating the claims of these various forms of segmental 

 divisions to rank as the primitive, the time of their respective 

 appearance in the developmental history will be significant. 



