The Reactions of the Vertebrate Embryo to Stimulation etc. 557 



rounding tissues, but few will be ready to admit with him tliat in 

 ali cases it is pos8Ìble to determine with accuraey the indivi- 

 duai connection of these protoplasmic spans with the intra-medul- 

 lary cells. 



The view recently expressed by Held (Verh. Anat. Ges. 20. Vers. 

 1906 pag. 185 — 202) to the effect that at the time when the first 

 motor and sensory nerve roots becorae visible these plasma bridges 

 already forni such an intricate and extensive network as to bafFle 

 :iny attempt at tliis comparatively late period in development of 

 determiniug- the cells from which they orij:inally S[)rang is more in 

 accord with the results of the present series of observations. Five 

 possible sources of origin have been assigned to these structures: 

 1) the cells of the neural canal; 2) those of the myotome; 3) both 

 of these groups have been said to take part in their formation; 

 4) the cells of the mesenchyme and 5) those of the chorda. Although 

 Held seenis to thiuk that the latter may contribute material for 

 their formation, I have seen no evidence which would seem to suj)- 

 port this view. Unfortunately it is impossible to pick out the in- 

 dividuai cells which actually take part in the formation of these 

 bridges so that the whole question is stili open for further investiga- 

 tion. There are a number of reasons why it seems probable that 

 these structures are sometimes of poly- and not of mono-cellular 

 origin. There is no direct ocular evidence that only one celL is 

 immediately concerned in their production, while in many sectious, 

 in the area« to be described the protoplasmic extrusions seem to 

 be so extensive that the idea of considering thera to be the pro- 

 ducts of a single celi is scarcely tenable. 



In the section from which the drawing represented in Figure 3, 

 Piate 23, was made it at once becomes apparent that the dimensions 

 of the bridge have attained such magnitude, particularly in breadth, 

 as to render it unlikely that this mass has during its development 

 been throwu out from a single celi. There are two paths within 

 the cord which I have closely studied, with the object of determin- 

 iug if possible the exact boundaries of the cells and the relation 

 they bear to the surrounding matrix. The first is the triaugle, 

 already described as forming the base of the span that extends 

 from the point of emergence of the ventral roots to the myotome, 

 and the second is the rim surrounding the centrai canal which at 

 certain stages and localities contains few if any nuclei. In both 

 places it is impossible to determina with accuraey the exact cells 



