Johnston, Mo7'phology of the Head. 2 1 1 



cause just mentioned. It is a serious question, however, 

 whether this influence would have been sufficient to determine 

 the course of the root in more primitive forms where the tracts 

 in question are less highly developed. In Petromyzon, although 

 the tracts between the inferior lobes and the cerebellum are very 

 small, the ascending and descending tracts between the tectum 

 and the oblongata are relatively larger and more conspicuous 

 than in Acipenser. There is good reason for thinking that 

 these tracts are among the most primitive and fundamental in 

 the brain, so that their influence would be felt in the lowest 

 forms. It should also be noticed that on account of the late 

 appearance of the trochlearis we may suppose that these sensory 

 tracts may have been already laid down before the fibers of the 

 nerve grew out. 



It may be mentioned here that the ventral decussation of a 

 part of the fibers of the third nerve may have been brought 

 about in a similar way. The nucleus of III lies among the 

 bundles of the ansulate commissure and in Petromyzon, where the 

 decussation of the nerve is large and conspicuous, the position 

 of its fibers (68, Fig. 24) suggests that their course may have 

 been determined by the path of least resistance offered by fiber 

 tracts already laid down. 



p. The absence of 7iejve roots from one or more hind brain seg- 

 me?its and the determination of the netiromere concerned. 



The only reference which I have seen made to the blank 

 neuromere (Sec. 4 above) as having special segmental signifi- 

 cance is by Neal (96, 97) who uses it in support ot the hypothe- 

 sis that one branchial segment has been supressed, the hyoid 

 arch representing two arches and this neuromere being de- 

 prived of nerve roots in consequence. This hypothesis was 

 first brought forward by vanWijhe and has been supported by 

 Miss Platt (106) and Hoffmann (127). Koltzoff {jG) strongly 

 argues that there is no ground in the development of the meso- 

 derm and the gills for the supposition that a branchial segment 

 has been suppressed. The writer is unable to see the advan- 

 tage of the hypothesis of a lost gill. No reason or cause has 



