560 Stewart Paton 



embryo, mauy points of contact between the periphery and central 

 nervous system were already distinctly outlined by these undifferen- 

 tiated tracts. To Hp^nsen imdoubtedly belongs the credit for having 

 years ag-o called attention to the importance of these connections, 

 and recently this investigator has repeated and amplified his original 

 descriptions (Die Entwicklungsmechanik der Nervenbahnen im Em- 

 bryo der Säugethiere, Kiel u. Leipzig 1903). Kerr, as already 

 indicated, has confirmed some of these observations in Lepidosiren. 

 The very decided view expressed by Hensen however against the 

 possibility of connections existing between nerve and connective 

 tisane cells does not seem to be justifiable particularly for the later 

 stages in the development of the embryo. The question is still an 

 open one, for at the time when the mesenchyme begins to creep in 

 between the myotome and neural canal it is extremely difficult to 

 determine the relations between the intruding tissue and the estab- 

 lished connections that have bridged the interval between the two 

 structures. At various points, at more advanced stages, one gets the 

 Impression from the manner in which mesenchymatous cells Cluster 

 about the primary bridges or plasmodesmata that the possibility of 

 material being thrown out from the fornier to supply the substance 

 necessary for the expansion and development of the latter, may be 

 a possibility. Kerr has described the manner in which the proto- 

 plasmic mass forming the rudimentary nerve spreads out over the 

 inner surface of the myotome and admits the possibility of its cover- 

 ing the whole of the inner surface. I have frequently observed a 

 similar condition but it has been practically impossible to determine 

 accurately the cells from which this mass may be ejected. This 

 fact alone should justify extreme caution in expressing a definite 

 opinion as to the number or identity of the Clements concerned in 

 its production. Even if it should be proven that the primary bond 

 of union between myotome and cord is the product of a single celi, 

 this does not in the least preclude the possibility of other cells 

 contributing material to permit of its expansion and growth along 

 the inner surface of the myotome. Prior to the appearance of the 

 ueurofibrils the nuclei within the cord are very sharply outlined l)ut 

 bere also it is quite impossible to determine the limits of the celi 

 boundaries. The whole mass seems to be welded together by the 

 interveuiug Strands of protoplasm which are continuous, not only 

 with the substance forming the neural base of the bridge at the 

 point of exit of the ventral roots, but also with the material enter- 



