426 CHARLES HILL, 



apex a groove which arises ontogenetically before the ridge. It seems 

 to me therefore that the ridge is a secondary modificatioo. 2) The 

 radial arrangement of cells mentioned by Orr is present only during 

 the stages of intrasegmental expansion of the neural walls to which 

 may be attributed the radial cytological condition. At least prior to 

 the expansion a radial arrangemenit is absent. 3) That the nuclei 

 should be "nearer the outer surface and approach the inner surface 

 only toward the apex of the ridge" is a condition that would, it seems 

 to me, also very naturally follow a neural intrasegmental expansion. 

 This nuclear arrangement is present only in late segmental stages. 

 While in later stages I find the three characteristics just mentioned, 

 there are present in early embryonic stages only the two following 

 criteria given by Orr, viz., 4) "The constrictions are exactly opposite 

 on each side of the brain" to which I would add : and in the same 

 transverse plane as the internal grooves with which they are continuous, 

 and 5) "On the line between the apex of the internal ridge and the 

 pit of the external depression the cells of the adjoining neuromeres 

 are crowded together though the cells of one neuromere do not extend 

 into another neuromere". To cover the case in Teleosts where the 

 rudimentary ridge is absent it would be necessary to modify the 

 statement as follows : The cells of the adjoining neuromeres are usually 

 crowded together though the cells of one neuromere at no time ex- 

 tend into another neuromere. To this should also be added Orr's 

 statement that "adjacent neuromeres present in some sections the 

 appearance of a septum extending from the pit of the external 

 depression to the summit of the internal ridge". 



Neural Segments in Teleosts. 



It is a pertinent question how far the description of the neural 

 segments in Teleosts, as presented in this contribution, confirms the 

 observations of other authors and what new facts have been considered. 

 To answer it attention must be directed to the literature. The first 

 recorded observation upon neural segments in Teleosts, known to me, 

 is by DoHRN in his Ursprung der Wirbelthiere und das Princip des 

 Functionswechsels, 1875, where he says (quoted from Hoffmann, in: 

 Bronn, Class. Ord. Thierreich, V. 6, Abtheil. 3, p. 1967): "If one ob- 

 serves the first stages of the embryonic formation of a Vertebrate, 

 for instance, a bony fish, one can scarcely escape the idea that this 

 is an animal made up of a large number of segments. The so-called 



