The Reactions of the Vertebrate Embryo to Stimulation etc. 555 



cbaracteristics yet mark the process of celi differeutiatiou witliin the 

 cord. The germinative cella of His are, as recent investigations 

 bave shown, to be regarded simply as mitotic cells. 



In the region of the ventral roots there is a marked bulging 

 forward of the protoplasm and in many sections a "bridge" about 

 4,u — bii in width similar to those shovpn by Kerr to exist in Lepi- 

 dosiren (On some Points iu the Early Development of Motor Nerve 

 Trunks and Myotomes in Lepidosiren paradoxa. in: Trans. R. Soc. 

 Edinburgh Voi. 41 1904) may be seen spanning the distance be- 

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

 myotome. One of these bridges is shown in Piate 23 Fig. 1. The 

 structure of the protoplasm of which these Strands are composed 

 seems to be similar in ali respects to that forming the general ma- 

 trix and does not at any point, except at the places to be descri- 

 bed later on, present evidence of a fibrillar structure. Less fre- 

 quently 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 gangliou 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 mo-. 

 ment when these bridges appear and the manner in which they are 

 formed necessitates the consideration of questions of fundamental 

 importauce. 



Are these structures the i)roduct of a single celi or do several 

 Clements coutribute protoplasm to span the interval between two 

 given points as widely separated as the periphery and cord? 

 Hensen's idea that these bridges are origiually thrown across from 

 one celi to another and theu as the embryo grows these threads are 

 pulled out to many times their originai 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 debatable point is merely in regard to the 

 manner in which they are formed. 



Neal (The Development of the Ventral Nerves in Selachii. in: 

 Mark Annivers. Voi. New York 1904), jn an interesting paper on 

 the development of the ventral uerves in Selachians, says that he 

 had been unable in any of bis sections to show the existence of a 

 protoplasmic connection "even of the raost 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 re- 



