1881.] Comparative Neurology. 23 
The likeness between the chain of ganglia in the leech and the 
spinal cord of Vertebrata has led many comparative anatomists 
astray in homologizing. A nearly similar chain of ganglia obtains 
in Vertebrata but situated ventrally from the vertebral column. 
This chain is a first system. The head ganglion, only, of the 
leech, as in most Invertebrata can be compared toa spinal. In 
Insecta and Myriapoda the superimposed secondary becomes 
more evident. An “ unpaired system” runs in the median line 
between and connected with the paired or primary system, typify- 
ing the more definite appearance of the medullary gray and its 
commissures below or back of the head. 
Todd and Bowman (pages 611 and 614, Vol. 111) use the fol- 
lowing words, which indicate an early recognition of the anatomi- 
cal fact without their having seen its connection or full import: 
“In the bee the cerebral (‘secondary’) ganglion is very large; 
from its anterior portion is given off two nerves which pass for- 
ward to the base of the antenne and have their origin well 
marked by a distinct ganglionic enlargement !” 
Todd dwells upon the importance of recognizing this distinct 
ganglionary enlargement, and repeats, “ The sensory nerves have 
ganglionic enlargements in the bee.” 
(This appearance of a third system is rare in Invertebrata, 
though the crab and Pterotrachea also may prove to be its pos- 
sessors.) 
The ganglionic swellings which on the sensory nerves of the 
bee distinguished it from most Invertebrata, in vertebrate types 
from Cyclostomes upward become more markedly developed. 
While both the first and second systems possess recognized 
afferent and efferent fibers, before being able to comprehend the 
relationships between systems or the process of projection forma- 
tion, we must consider whether some fundamental law does not 
underlie these series of relations which will better sohaneaicie for 
their creation. 
The typical segment is an animal whose nerve center lies mid- 
way between an afferent and efferent strand, thus: $. A series 
of such segments, if ununited, present this appearance : 
Le 
Ecc er! 
These segments could be correlated by a second fiber, which 
