244 Mr. A. S. Packard on the Asj)ects of the 



These facts relate to the histology and the histological 

 topography as well as the general morphology of the system in 

 question, and to the general relation of the viscera to the body- 

 walls of Arthropods as compared with Vertebrates. 



1. Histology. — There are but two histological elements in 

 the brain and spinal coid of Vertebrates, i. e. ganglion-cells 

 and nerve-fibres proceeding from them. In Worms (and 

 JVlollusks so far as known) and especially in the brain (pro- 

 cerebrum, as we may call it to distinguish it from the cere- 

 brum of Vertebrates) and other ganglia of Crustacea and 

 insects, besides these two elements there is a third substance, 

 the Punhtsubstanz, discovered by Leydig, and further de- 

 scribed by Uietl and Krieger, and forwiiich we would suggest 

 an English equivalent, the myeloid substance. 



2. Histohgical Topography — The arrangement of tlie gan- 

 glion-cells and other tissues in the ganglia of Arthropods is 

 not homologous with that of Vertebrates. In the brain or 

 any of the postcesophageal ganglia of Arthropods there is a 

 central mass formed of the myeloid substance, which is en- 

 veloped by a cortical layer of mostly unipolar ganglion-cells. 

 The fibres from the ganglion-cells pass into and emerge 

 again from the myeloid substance, which is a tangled 

 mass of minute fibrillar. The fibres from certain of the 

 ganglion- cells we have clearly seen to pass through or over 

 the myeloid substance and to form both the transverse com- 

 missures of the brain and also the two main longitudinal 

 commissures connecting the chain of ganglia. But the fibres 

 from the majority of the ganglion-cells appear, as Leydig 

 holds, to break up into the tangled mass of extremely fine 

 fibres, which, when cut through, presents a dotted or granu- 

 lated appearance. This myeloid substance remains unstained, 

 while the ganglion- eel Is readily stain by reagents. 



In the brain and other ganglia of vertebrates, on the other 

 hand, the ganglion-cells are internal, the fibres arising from 

 Tini-, bi-, or multipolar ganglion-cells passing outside. In 

 Invertebrates, at least in Arthropods, there is no " white " or 

 "grey " substance ; none such has been described by Leydig 

 or the later students of the central nervous system of 

 Arthropods. 



Histogenesis. — If we look at the genesis of the ganglia of 

 Arthro])ods, we see that they consist at first wholly of sphe- 

 rical cells, the fibres and myeloid substance being secondary 

 products, and their position is not homologous with that of 

 the ganglia in vertebrate embryos. The reader is referred 

 to fig. 246 in Balfour's ' Comparative Embryology,' vol. ii. 

 p. 343. The section of the spinal cord of a seven-days' chick 



