1897.] Cephalic Homologies. 939 
tions, and have done little more than attempt to present the 
problem critically. As regards the head, however, even theo- 
ries have hitherto failed us, and no hypothesis as to the evolu- 
tion of the vertebrate head from the Annelidan type has been 
brought forward, which could not be shown to encounter in- 
superable objections, or at least which appeared insuperable 
to those who were opposed to the Annelidan theory. The 
suggestion most widely known and discussed is that the in- 
vertebrate mouth disappeared, the oesophageal ring with the 
neighboring ganglia formed the vertebrate brain, and a new 
vertebrate mouth was evolved, according to Dohrn, by the 
fusion of two gill clefts in the ventral median line. 
[ask your consideration for an entirely different theory of 
the homologies of the vertebrate head. A morphologist must 
feel grave hesitation in assuming that such important struct- 
ures as the main lateral eyes and as the oral evagination 
should have totally disappeared during the evolution of Ver- 
tebrates, for they are an advancing type, not a degenerate type 
losing its organs. He must also feel grave hesitation in as- 
suming that main lateral eyes have been evolved in the Artic- 
ulates and Vertebrates without any genetic relationship. It 
is natural, therefore, to test the assumption that the articulate 
eyes and vertebrate eyes are phylogenetically homolgous, it 
being, of course, understood that the comparison excludes 
ocelli, accessory eyes in Annelids and the pineal eye of Verte- 
brates. We note at once that the visual sensory apparatus in 
both cases is epithelial in type, and is derived immediately 
from the ectoderm, and further that in both cases the sensory 
apparatus is directly connected with nervous substance, in 
Articulates the so-called optic ganglion, in Vertebrates the ner- 
vous tissue of the retina, and finally that in both cases there 
runs from the optic apparatus a fibre tract, which is not a 
nerve, but a commissure, that is to say, a differentiation of a 
part of the central nervous system itself; this tract we calla 
part of the oesophageal commissure of the Articulate, and the 
optic nerve of the Vertebrate. We are thus lead to the sup- 
position that eyes and optic nerves represent in Vertebrates 
the main eyes, the supra-oesophageal ganglia, and the oseopha- 
