THe Microscope. . 997 
gill slits, and many other points of structure are fully as im- 
portant. 
The statement that the nervous cord of arthropods at first 
appears as a series of disconnected ganglia and hence cannot be 
homologized with that of vertebrates, is at once negatived by 
the fact that in both insects and crustacea the nervous system 
usually arises as longitudinal ridges which later become seg- 
mented into ganglia. The earlier stage seems to have escaped 
Dr. Packard. Even were he correct the two would still be ho- 
mologous, the metamerism of the mesoblast affecting the one at 
an earlier stage than the other. 
In the last section of his article our author seems to have 
carried his homological skepticism to the greatest extreme and 
to be decidedly unfortunate in some of his comparisons. For 
instance in denying the homology of the eyes of vertebrates and 
invertebrates he has mentioned as proving his point their only 
similarities. His text runs thus: ‘The eyes of arthropods are 
not truly homologous with those of vertebrates; the cornea is 
simply a number of epithelial cells, while in vertebrates the eye 
externally is an ingrowth of the epiblast.” What, we would 
ask, are the epithelial cells if not epiblastic? Dr. Packard 
doubts even the homology of the heart in the two groups, and 
also the relations of the viscera to the body walls. Let us see 
what are the facts in the case. In both groups the mesoblast 
arises as an outgrowth from the lips of the blastopore or from 
the archenteron, though on account of a large amount of food 
yolk this origin is frequently obscure. In both groups the mes- 
oblast splits into two layers with the body cavity between. The 
inner of these two laryers, the splanchnoplure, forms the inner 
wall of the body cavity, while the outer forms the principal 
part of the body wall, muscles, bones, etc. In both vertebrates 
and arthropods from this outer layer, the somatoplure, the heart 
is formed. Need the homology be closer ? 
In this article it has not been the purpose to support any 
theory, but merely to show the lack of cogency in the reasoning 
of our author. Within the last ten years vast strides have been 
made toward the solution of morphological questions, and when 
one considers the important bearings of evolution, for to such 
the question belongs, he is apt to rebel any religation of these 
subjects “ rather to metaphysics than to pure science.” N. 
