556 Stewart Paton 



concile tbis statement with the results of ray own observations, as 

 well as those of other investigators wbo bave repeatedly observed 

 tliese bridges in Selachians. At tbe time wben Neal's investigations 

 were conducted tbere was uo metbod of staining whieh was capable 

 of ditferentiatiug tbe componeut parts of the neuraxon, and it is not 

 at all improbable tliat tbe structiire wbich he bad reason to believe 

 was tbe groAving end of a neuroblast was only an undifferentiated 

 protoplasmic band or bridge. In tbe second place tbe metbod of 

 fixation nndoubtedly bas sometliing to do witb tbe failure to detect 

 the existence of tbese structures, wbich are mucb more easily de- 

 monstrable in sections fixed in corrosive-acetic or neutral for-mol, tban 

 tbey are in Solutions containing picric acid. For tbe reasons men- 

 tioned tbe structures represented by Neal as neuraxons cannot be 

 accepted as such witbout furtber proof. Probably tbe long processes 

 depicted by tbis investigator as beiug projected from medullary cells 

 are in reality niade up of two componeuts: a short process and tbe 

 long undifferentiated protoplasmic Strand or bridge witb wbich it is 

 apparently fused so as to give , in specimens stained by certain 

 metbods, tbe appearance of a single long process. In Piate 23 

 Fig. 1 one of tbese bridges is represented, the proximal side of 

 wbich bas fused witb tbe matrix of tbe cord while the distal is 

 united witb that of tbe myotome. 



A similar conditiou is also depicted in Fig. 2. At later stages 

 one may find connections present between tbe cord and group of 

 cells, wbich eventually form tbe spinal ganglia, and between tbe 

 latter and the peripbery. Furtber tbere is abundant opportunity to 

 study tbese plasmodesmata (Held) in tbe rcgion of the Cranial nerves, 

 where undifferentiated links of protoplasin frequently unite tbe exist- 

 ing ganglionic masses eitber witb tbe central nervous system or 

 with tbe peripbery. In tbe case of tbe Oculomotorius and Trochlearis 

 tbe existence of tbese bridges is very problematic. 



A number of investigators bave already directed attention to 

 tbe presence of tbese primitive protoplasmic bands in connection 

 witb tbe anterior nerve roots. In 1888 Dohrx, in describing tbe 

 origin of tbe ventral roots in Selachians said that tbe tirst appea- 

 rance of tbese structures was characterized by tbe extrusion of a 

 bomo.i;eneous structureless protoplasmic efifusion from tbe spinal cord. 

 Aniong more recent observers it is only necessary to quote Fkoriep 

 (Verb. Anat. Ges. 18. Vers. 1904 pag. 12) as reaffirming tbe existence 

 of tbese structureless homogeneous bands uniting the cord and sur- 



