1897.) Cephalic Homologies. 931 
the former, while a pharynx arose by the concresence of 
branchiate crustacean legs, the spaces between the legs becom- 
ing gill clefts. There are two fatal objections to this theory. 
I consider it a first and sufficient objection that it sets aside 
the evidence that the primary germ layers, ectoderm and en- 
toderm are homologous throughout the metazoa. The evidence 
on this point is overwhelming. Gaskell’s theory leaves a 
heart between the pharynx and the nervous system, so that to 
save his theory he has to make two supplementary hypotheses, 
first, that the crustacean heart disappears, and second, that a 
new or vertebrate heart is formed by the tips of the legs where 
they grow together. It is, indeed, difficult to give morpholog- 
ical credence to these two hypotheses. If Gaskell’s hypotheses 
were true, we should expect to see some trace in the ontogeny 
of Vertebrates of the development of the pharynx by the con- 
crescence of solid outgrowths, but, as well known, the pharynx 
is a series of hollow entodermal evaginations. As regards the 
neuron, Gaskell supposes that the ependyma represents the 
epithelium of the crustacean intestine, while the nervous mat- 
ter represents that which collects in the walls of the crustacean 
intestine. This view, which is essential to his hypothesis, 
overlooks the fact that in Vertebrates the same epithelium 
produces both ependyma and nerve cells. .Very numerous 
other objections can be raised, and consequently I am ready 
to go so far as to maintain that the acceptance of Gaskell’s 
theory is impossible. 
B. Patten’s theory seeks in the Arachnid type the ancestral 
form of Vertebrates. Under Arachnids Patten includes the 
Arachnida sensu strictu, and the Trilobites and Limulus also. 
It is noteworthy that the original articles by Patten and Gas- 
kell appeared in the same number of the Quarterly Journal of 
Microscopical Science, (August, 1890). I have often wondered 
whether the juxtaposition was due to editorial design. Patten 
rests his case chiefly upon comparison of the nervous system, 
and makes only somewhat uncertain and tentative suggestions, 
as to the three serious difficulties offered by vertebrate char- 
acteristics, namely, 1, the disappearance of the invertebrate 
and origin of the vertebrate mouth; 2, the origin of the 
