24 H. V. NEAL 



Kerr ('04) to exist in Lepidosiren may be seen spanning the distance 

 between the edge of the medullary substance and the inner border of 

 the myotome. The structure of the protoplasm of which these strands 

 are composed seems similar in all respects to that forming the general 

 matrix and does not at any point, except at the places to be described 

 later on, present evidence of a fibrillar structure. Less frequently 

 strands of protoplasm are seen in other localities, as for example at 

 the point where later in the development of the embryo processes of 

 giant ganglion cells (Beard) emerge from the cord, or where the matrix 

 surrounding the cells of the ganglionic masses is in contact with the 

 periphery. Any attempt to determine the moment when these bridges 

 appear and the manner in which they are formed necessitates the con- 

 sideration of questions of fundamental importance. 



Are these structures the product of a single cell or do several ele- 

 ments contribute protoplasm to span the interval between two given 

 points as widely separated as the periphery and cord? Hensen's idea 

 that these bridges are originally thrown across from one cell to another 

 and then as the embryo grows these threads are pulled out to many 

 times their original length, is an exceedingly ingenious and suggestive 

 hypothesis but has not yet been proved. There cannot be any ground 

 however for doubting the existence of these structures. The clebatable 

 point is merely in regard to the manner in which they are formed. 



Fui^her on he says: 



Neal ('03) in an interesting paper on the development of the ven- 

 tral nerves in Selachians, says that he has been unable in any of his 

 sections to show the existence of a protoplasmic connection 'even of 

 the most attenuated kind between the somite and the neural tube 

 before the first neuraxon makes its exit from the neural tube.' At 

 first it was difficult for me to reconcile this statement with the results 

 of my own observations as well as those of other investigators who 

 have repeatedly observed these bridges in Selachans. At the time 

 when Neal's investigations were conducted there was no method of 

 stainmg which was capable of differentiating the component parts of 

 the neuraxon, and it is not at all improbable that the structure which 

 he had reason to believe was the growing end of a neuroblast was only 

 the undifferentiated protoplasmic band or bridge. In the second place 

 the method of fixation undoubtedly has something to do with the fail- 

 ure to detect the existence of these structures which are much more 

 easily demonstrable in sections fixed in corrosive — acetic or neutral 

 formol than they are in solutions containing picric acid. For the rea- 

 sons mentioned the structures represented by Neal as neuraxons can- 

 not be accepted as such without further proof. 



Paton goes on further to say (p. 556) that: 



Probably the long processes depicted by this investigator as being 

 projected from medullary cells are in reality made up of two com- 



