1884.] Aspects of the Body in Vertebrates and Arthropods. 857 
through presents a dotted or granulated appearance. This myel- 
oid substance remains unstained, while the ganglion-cells readily 
stain by reagents. 
In the brain and other ganglia of vertebrates, on the other 
hand, the ganglion cells are internal, the fibers arising from uni, 
bi or multipolar ganglion-cells passing outside, In invertebrates, 
at least arthropods, there is no “ white” or “gray” 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 
arthropods, we see that they consist at first wholly of spherical 
cells; the fibers 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 Bal- 
four’s Comparative Embryology, Vol. 11, p. 343. The section of 
the spinal cord of a seven days’ chick there figured, shows that 
the cord is early differentiated into the internal gray mass, consist- 
ing of round cells, enveloping the spinal canal, while the cortical 
white substance or column surrounds the mass of ganglion-cells. 
In the annelidan worms and the arthropods, the embryonic gan- 
glion is a much simpler structure, consisting of a mere mass or 
ball of ganglion-cells, with incipient fibers passing from them. Cer- 
tain of these fibers grow longer, forming the commissures, trans- 
verse and longitudinal, connecting the ganglia. At first, then, the 
nervous system of the higher worms (those with a ganglionated 
chain) and arthropods, consists of a series of disconnected ganglia, 
which eventually become connected by secondary products, the 
commissural fibers. The fact that in worms the brain is at first 
separate from the rest of the ganglia, as stated in Balfour's Em- 
bryology (I, p. 291), is not of particular significance since all the 
ganglia, at least in Crustacea and insects, are at first disconnected 
from each other. 
Embryology appears to give no countenance to the view held 
by some authors that the brain of an arthropod may represent the 
nervous system of the vertebrate, and the post-cesophageal chain 
of ganglia the sympathetic system of the vertebrates. 
There seems to be a unity of plan, so to speak, in the develop- 
ment of the nervous system of the arthropods, and how radically 
different that is from the mode of genesis of the vertebrate ner- 
vous system may be seen by reference to Balfour’s work (u, 
