The Reactions of the Vertebrate Embryo to Stimulation etc. 559 



It Ì8 not difficult to find outside of the nervous system condi- 

 tious that are analogous to those occurring within the neural canal. 

 In studying the histogenesis of the retina, Weyse & Burgess (Amer. 

 Naturai. Voi. 40 1906) have called attention to the peculiar manner 

 in whieh the reticular layers are formed from the cytoplasm without 

 any extrusion of nuclear substance, while the rods and cones seem 

 to develope from threads extending between the pigment layer and 

 the external limiting membrane. 



In the development of the skeletal as well as cardiac museles 

 a fusion of the cells quite analogous to that observed in the 

 nervous system has been described by Godlewski (Arch. Mikr. Anat. 

 60. Bd. 1902 pag. Ili) whose results have in the main been recently 

 confirmed by Marceau (Ann. Se. N. [8] Tome 19 1904). On account 

 of this remarkable similarity of the two processe» it is worth while 

 recalling Godlewski's account of the changes as they occur in the 

 myotome. ßeferring to the disposition of the cells in the more 

 cranially situated portions of the myotome in a rabbit embryo of 

 11 days, he says, that in this region the celi boundaries tend grad- 

 ually to become more and more indistinct. A celi syncytium is 

 formed, which increases in extent not as in the ordinary manner by 

 the growth of the celi body, nor by nuclear division, but by the 

 fusion of cells possessing similar structural characteristics. 



It is not my j)urpose in the present paper to attempt to describe 

 more in detail the various poiuts where a so -called syncytium or 

 extrusion of protoplasm is formed, but merely to direct attention to 

 the importance of studying the origin and development of these 

 protoplasmic effusions on account of the relation they bear to the 

 subsequent fuUy differentiated tracts for the conduction of nervous 

 Impulses. After studying carefully sections fìxed and stained by a 

 great variety of methods, I am satisfied that these apparent ex- 

 trusions, quite free from nuclei, canuot he the result of an imperfect 

 technique. This was the idea which suggested itself to my mind 

 when these areas first attracted my attention ten years ago in study- 

 ing the histogenesis of the cerebral cortex in the pig. At that time 

 the true significance of this condition was overlooked and these 

 apparent early connections between the cells were considered to be 

 mere artefacts and, consequently, were studied very superficially. 

 The importance of the subject was again presented in a very strik- 

 iug manner when it was noticed that, at the time the first physio- 

 logical reactions for external Stimulation began to appear in the 



Mitthoilnngen a. d. Zool. Station zn Neapel. Bd. 18. 36 



