34 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
hemispheres, optic vesicles, and olfactory lobes, and there is a 
corresponding continuation of the skeletal axis by the development 
of the trabecule and the skeletal structures which arise in con- 
nection with them. 
In the head I have just described there are developed all the 
primitive vertebrate structures, but there is a complete absence 
of most of those peculiar to the craniota. The failure in develop- 
ment could not have been produced by an occlusion of blood- 
vessels, since the ground plan must have been laid down before any 
blood-vessels were formed ; and the only conclusion which seems 
tenable is that there has been some alteration of the potentiality 
of the developing cells of the ovum, so that they have completed 
their evolution when they reached the stage corresponding to the 
adult development of the remote ancestor. 
The ordinary form of Cyclopean is probably the result of a 
compromise between the present extreme form and the normal 
development, 
