8.S0 



The only other view is that of His, who, as is well known, con- 

 siders them to arise from a peculiar portion of epiblast called the 

 „Zwischenstrang". I will not now enter at length into this question; 

 as the result of ray researches in many Vertebrates I can show that 

 the Zwischenstrang of His just fails to take any share in the 

 formation of the ganglia, and that as a matter of fact in all 

 cases the latter are neither formed from His' structure nor as out- 

 growths of the spinal cord, but that they owe their formation to the 

 inner epiblast between the extreme dorsal or neural limit of the 

 Zwischenstrang of His and the „Anlage" of the spinal cord. The in- 

 vestigation — an exceedingly delicate one — was made on embryos 

 submitted to the action of Flemming's chrom-osmic acid mixture, 

 and as proving that the appearances described are not due to the 

 reagent used, I may here mention that the development is seen to be 

 essentially the same in Elasmobranch embryos killed by corrosive sub- 

 limate. Indeed, when I look at the preparations of the latter group, 

 I can only marvel that no one has seen before the first and in- 

 dependent origin of cranial and spinal ganglia. 



In three very favourable Chick embryos with no somites, six somites, 

 and ten somites respectively all the earliest stages may be seen. I will 

 here only describe the one with six somites. In this embryo in a 

 section thro' the primitive streak no trace of the formation of the ganglia 

 can be made out. The epiblast is thick and composed of several 

 layers of cells, and this thickening extends outwards far beyond the 

 limits of what will be the spinal cord. There is here a very slight de- 

 pression forerunning the invagination of the latter. Further forwards 

 the epiblast has the same general appereance as in the preceding 

 section, but the invagination is more marked, and one can distinguish 

 more clearly the limits of what will be the spinal cord. Outside of 

 each end of these limits, in the reentering angle formed by the spinal 

 cord Anlage and the rest of the epiblast, a somewhat cone shaped loo- 

 sening of certain cells of the inner epiblastic layer is observable. 

 These cells show a tendency to accompany the future dorsal margin 

 of the spinal cord upwards, and thus to get detached from the rest 

 of the epiblast. 



Further forwards this process becomes still more marked, and 

 concomitantly with the advancing invagination this group of cells leaves 

 its connection with the thick lateral epiblast, and becomes more and 

 more carried up in the reentering angle described above. 



When the neural folds are on the point of closure, one finds the 

 dorsal epiblast above the true lips of the spinal cord composed of a 



