296 THe MIcRoscoPE. 
“ Mesenterial filiaments of the Alcyonaria” (Mittheilungen a. 
d. Zool. Sta. Neapel v. 1884). 
It now appears that all morphologists, with the exception 
of Dr. Packard, admit the homology of the nervous system 
throughout the whole of the Metazoa. In all it arises as a por- 
tion of the epiblast which eventually sinks into the underlying 
tissues. Further, in all forms as far as known, its method of 
formation can be reduced to one type, a cord of cells arising on 
either side of the blastopore. In all segmented animals there 
is a further similarity to be noticed. It consists of a series of 
ganglia metamerically arranged. These points conclusively 
prove the homology so far as they go and the only difficulty is 
one not noticed by Dr. Packard. In the arthropods the alimen- 
tary tract passes through the nervous system and in the verte- 
brates it does not. For our purposes it matters not which ex- 
planation we accept; that of Semper and Dohm, who would 
homologize the hypophysis cerebri with the invertebrate mouth, 
or that of Sedgwick, who claims that in the vertebrates the pre- 
oral portion of the invertebrate nervous system has disappeared ; 
or lastly that of Hubrecht who imagines that the lateral nerve- 
cords of the Nemerteans have coalesced on the ventral surface 
of the arthropods and on the dorsal of the vertebrates. 
Dr. Packard lays great stress upon the fact that in the nery- 
ous cord of arthropods there is an element (which he calls the 
myeloid substance) which is entirely lacking in the vertebrates, 
a point which is not beyond dispute. Homology does not de- 
mand exact equivalence in minute detail. The arm of a man 
and the pectoral of a fish are greatly dissimilar, but would Dr. 
Packard deny their homology ? 
Another point deemed of great importance is the fact that 
in the vertebrates the neural vord contains a longitudinal canal 
which is lacking in the other group. If Dr. Packard will con- 
sult a paper by Mr. Adam Sedgwick (Proceedings of the Cam- 
bridge Philosophical Society, iv. p. 325) he will find some,valu- 
able hints upon this point, which go to show that the vertebrates 
left the main invertebrate stock before the great majority of the 
invertebrates. As used by Dr. Packard the fact proves noth- 
ing. It is not alone the presence of this cord which gives the 
ascidian its place among the vertebrates ; notochord, endostyle, 
