VERTEBEATES FROM A CRUSTAOEAN-LIKE ANCESTOR. 437 



Laboratory under my direction, and I will at present only 

 draw attention to one or two points. In the first place, we see 

 that the nerve-cells of the central nervous system of such an 

 animal as a crayfish vary in size, being divisible into giant- 

 cells, large cells, and small cells ; so also in the Aramocoetes we 

 find the same three classes of cells. In the Aramocoetes the 

 giant-cells are connected with the large Miillerian fibres which 

 constitute a system of longitudinal paths in the nervous 

 system itself; the large cells give origin to the series of seg- 

 mental spinal and cranial nerves ; and the small cells form the 

 closely set masses of '^ beerenformigen '' cells which constitute 

 the chief part of the spongy portion of the grey matter in 

 the upper portions of the central nervous system. 



In the crayfish the large cells are in connection with the 

 outgoing segmental nerves ; the small cells form closely set 

 masses of cells which bear a strong resemblance to the cells 

 of the spongy portion of the grey matter, and are connected 

 with the reticulated substance (Punctsubstanz) rather than 

 directly with outgoing nerves. Ahlborn's description of the 

 arrangement in the Petromyzon of these small cells, in rows 

 like bunches of grapes on a stalk, and the appearance of them 

 as I myself have seen them, is strikingly illustrated in Patten's 

 fig. 7, on the plate illustrating his paper (25) on the develop- 

 ment of the eyes of Vespa, &c. Whether the giant-cells in 

 the Arthropod nervous system resemble those in the Aramo- 

 coetes, and are connected with a system of longitudinal fibres 

 in the central nervous chain itself, I do not as yet know, 

 although I think it very probable that such will be found to 

 be the case. 



Again, the similarity in the extent of this small-celled 

 group is very striking. In the spinal cord of the Aramo- 

 coetes, and indeed of all Vertebrates, the small cells of the 

 grey matter are but few, and confined mainly to the region 

 of the posterior horn ; when, however, we reach the higher 

 regions of the central nervous system we see how greatly this 

 particular class of cell increases in number, forming the main 

 feature of the grey substance ; we see how masses of these 



