The Reactions of the Vertebrate Embryo to Stimulation etc. 563 



In Fig. 3, Piate 23 it may very readily be seen that the fibrils 

 lying in the apex of the ueuroblast (a) are distiuctly diiferentiated 

 from the envelope of protoplasm which forms the process. No such 

 envelope surrounds the fibrils lyiag within the bridge (e). By 

 studyiug- the sections in front of and behind the piane of section, repre- 

 sented in this drawiug, it becomes obvious that the bündle of fibrils 

 whose thick^ ends rests at the point (e) in the bridge is not con- 

 nected with those in the celi {a). In imperfectly diiferentiated spe- 

 cimens artificial links are frequently formed by the Silver deposit 

 betweea the ends of two such bundles which histologically are quite 

 distinct, and in this way silhouettes of cells with elongated processes 

 are often obtained. 



Ramon y Cajal first called attention to the development of the 

 primitive neurofibrils at some distanee from the cells with which 

 they were supposed to be connected and, as a possible explanation 

 of the phenomenon he suggested that as the parts of the protoplasm 

 furthest from the nucleus were the oldest, therefore they were the 

 first to be differeutiated. Naturally the correctness of the inference 

 depends upon the proof that the fibrils observed are definitely 

 connected with individuai cells. Methods capable of differentiating 

 the tissues and not giving nierely silhouettes, the result of deposi- 

 tiou of Silver, bave so far fyiled to show the existence of these links. 

 If however in spite of this fact the Suggestion is made that the 

 primary fibrils, lying for example in the myotome in embryos of 

 5 mm. in length, are merely the -terminal portions of processes be- 

 louging to intramedullary cells, then the difficulty arises of explaiuing 

 how it is that these fibrils He in a territory into which the processes 

 bave not yet penetrated. If the affirmation is made that these fibrils, 

 lying at a cousiderable distanee from the ventral horns, are in re- 

 ality connected with the unstained intramedullary tracts I am not pre- 

 pared to definitely accept nor to deny the validity of the objection. 



Should incontrovertible evidence be furnished demonstratiog the 

 continuity between the medullary celi processes and these extramedul- 

 ary fibrils it would not materially conflict with any of the other facts 

 observed. It is necessary to remember however, that no reliable 

 method of staining has yet been employed that is capable of de- 

 monstrating the presence of processes in the vicinity of the distai 

 ends of these primitive filaments. 



In Fig. 5, Piate 23 the degree of differentiation that exists be- 

 tween the fibrils and the protoplasm of the processes is represented. 



