THE DEVELOPMENT OF THE NERVOUS SYSTEM. 481 



resemble those exhibited in my sections, but I am unable to 

 regard the large cells as a layer of neuroblastic cells, although 

 the nervous system is undoubtedly differentiated from this 

 layer, and I think it probable that each ganglion is formed 

 from a small number of these cells. Many of the large cells, 

 perhaps the greater number, are probably the origin of the 

 muscles, and originate from the ccelomic sacs (PI. XIV., Fig. 2) ; 

 and certainly, in the earlier stages of the development of the 

 Blow-fly embryo, it is not possible to distinguish the cells in 

 which the ganglia of the ventral chain are first seen imbedded, 

 from those which subsequently form chains from which the 

 muscle fibres of the larva are developed. 



Hatschek, Will, Heiden, and others, have described a median 

 inflection of the cutaneous epiblast, between the two lateral 

 chains of ganglia in various Arthropods. Kowalevski did not 

 find any such inflection in Hydrophilus, nor have I seen any- 

 thing of the kind in the Fly-embryo. It appears to me, from 

 the very advanced condition of the embryos figured by Hatschek, 

 that the involution is secondary, and is perhaps a rudimentary 

 entosternum. To those who accept Dohrn's views, the existence 

 of such an involution is of primary importance, if it is developed 

 previously to the ganglia and really enters into their formation ; 

 but the evidence derived from the study of the Blow-fly embryo 

 appears to me to be quite incompatible with such a supposition. 

 Balfour, commenting on Hatschek's statements, says: ' It is 

 probable that Hatschek was entirely mistaken as to the entrance 

 of a median element into the constitution of the ventral cord;' 

 but in this I think Balfour is wrong. There is considerable 

 evidence that a median element is present in certain stages of 

 development ; even at an early stage, a median inflection of 

 the coslom is seen in certain of my sections, with a distinct 

 mesoblastic wall. It is probably from this that the tracheae 

 and connective elements, which exist in the median septum 

 of the neuroblast of the larva, originate. 



At a later period I have obtained excellent sections, showing 

 the ventral cord (as represented in PI. XIV., Fig. 2) entirely 

 separated from the surface epiblast. In some of these sections, 



