I lo WHEELER. [Vol. VIII. 



figures a very similar budding-zone of ectoderm cells in the 

 Decapod Astacus ('86). That a portion of the ectoderm cells 

 thus budded forth in these forms goes to form the nervous 

 system, admits, it seems to me, of very little doubt. The 

 clustered condition of the neuroblasts in the young germ-band 

 of Xiphidmm may be due to the rapidity with which the 

 germ-band grows in length and breadth; the original regular 

 arrangement of the cells being thereby disturbed and not 

 re-established till a somewhat later stage. 



Although I have maintained a phylogenetic connection of 

 the insect neuroblasts with the neural cell-rows of Annelids, I 

 admit that they may be regarded from an entirely different 

 standpoint, vis. as having arisen independently in insects by 

 a process of precocious segregation ; but it should be noted 

 in this connection that it is just the oldest Pterygota, the 

 Orthoptera, which show this segregation most clearly, while in 

 the more recent forms {Coleoptera, etc?) the process is more 

 obscure. 



A comparison of the histogenesis of the insectean with the 

 histogenesis of the vertebrate nervous system brings out some 

 interesting analogies. The neuroblasts may be compared with 

 His' Keimzellen which divide by caryokinesis to form his neuro- 

 blasts. These are directly converted into ganglion-cells by 

 sending out axis-cylinder processes. They correspond, there- 

 fore, to the daughter-cells of the insect neuroblasts, which 

 are likewise converted into ganglion-cells. The " Keimzellen " 

 of vertebrates differentiate close to the central canal, which is, 

 of course, morphologically the outer surface of the cord, just as 

 the Arthropod neuroblasts lie at the surface of the cord. In 

 both groups the daughter-cells appear to be budded off in the 

 same direction morphologically; though in vertebrates the gan- 

 glion-cells migrate, while in insects they are pushed inwards by 

 their newly proliferated sister-cells. The early separation of 

 the neural ectoderm in vertebrates into Keimzellen and susten- 

 tacular tissue (spongioblasts of His) is paralleled in insects by 

 the precocious splitting of the same germ-layer into neuro- 

 blasts and dermatoblasts, the latter giving rise to supporting 

 tissue in the form of neurilemmata. 



