NOTES ON THE ANATOMY OF A FORM OF CYCLOPEAN, 33 
In seeking for an explanation of Cyclopeans and various other 
monsters, we constantly are met by the reply—an arrest of develop- 
ment. But this merely leads to the further question—why is the 
development arrested? When there has been failure of development 
of a part through embolism or thrombosis in the vessels supplying 
the part, the arrest is satisfactorily accounted for. But if we find 
that the cells of a part have not multiplied in a certain axis or line 
of growth as they normally should, no pathological condition of the 
vessels can account for it. 
For some time I have been led to think that atavism plays a 
larger part in the formation of monsters than has commonly been 
supposed. In the paper which I read before this society “On a 
Monstrosity of the Common Earthworm, Lwmbricus terresiris, 
L.,”* I suggested the connection between double monsters and the 
process of development in Lumbricus trapezoides, Dugés, where 
normally two embryos are formed from one ovum, and that possibly 
in double monsters we had an attempted reversion to an ancestral 
mode of development. The development of an additional toe on 
each foot of the horse is an undoubted case of atavism, as pointed 
out by Prof. Marsh, When we meet with a case where a 
branchial cleft is left patent in the neck, it is customary to look 
at the case as one where development has been arrested in the 
part at the stage corresponding to the adult fish. But if, in the 
case of the horse, we admit that Nature at times develops one 
showing some of the characters of the three-toed ancestor, we may 
also admit that at times there are developed in the mammalian 
young some of the still more remotely ancestral characters of the 
fish. 
In the Cyclopeans we find preserved still more primitive 
characters. Between the Acrania, with its representative the 
Amphioxus, and the Craniota or head-bearing vertebrates, there is 
an immense gap in which we have no link at present known, un- 
less, perhaps, it be found in some of the anomalous Devonian fishes. 
The head of the higher vertebrates probably represents nine 
segments of the primitive form, with the additional development 
of parts not represented in the rudimentary vertebrates. The 
cerebro-spinal axis is continued forward by the addition of cerebral 
* Trans. Nat. Hist. Soc. Glasg., vol. ii. (1887-88), p. 203. 
