viii INTRODUCTION 



all the other structures except the bony framework have been laid down in their final form, 

 and already exhibit their adult relations. Each specimen examined thus provides us in small 

 compass with a compendium for its type, the morphology and histology of which can be studied 

 at the same time and from the same series of sections. (3) The vertebral column is of relatively 

 great length, while the limb masses are small. Accordingly, in the most interesting types of 

 monstrosity, namely those in which there is axial union of two or more embryos, we may 

 expect to find the characteristics of such union exhibited in simplest form. The law that 

 homologous structures become joined together, always holds good, and the full series of double 

 and multiple forms in fishes provides us with examples in which " compositeness," due to primary 

 or secondary fusion, is exhibited by practically every organ and structure in the body. (4) In 

 the great majority of fishes segmentation is partial ; we can speak of a circumscribed blastoderm 

 resting on the yolk mass and giving rise at one part to the rudiment of the embryo. In these 

 points there is correspondence with avian and reptilian development, and with those characteristics 

 of mammalian ontogeny which indicate an ancestral highly yolked condition of the ova. Accordingly 

 we find that in their major abnormalities the fishes show surprisingly close relationships even 

 with the mammals, 



From the general point of view the most interesting teratological questions are those which 

 are concerned with origin and cmisation. A great deal of the data necessary for forming properly 

 grounded views on those questions is still in process of accumulation. Meantime, I would venture 

 to put forward the following propositions, from my reading of the available evidence, particularly 

 as it has presented itself to me in connection with the teratology of fishes. The underlying 

 ideas have long been part of the currency, if not of the accepted currency, of biological thought. 



I. Each of the recognised types of monstrosity, major as well as miiwr, can arise in a 

 spontcmeous or autogenetic manner, by abrupt germinal variation. These terms are used with the 

 meaning that (a) the results are not due to the action of environmental factors on the 

 germ-cells directly concerned, or on the embryo itself ; (b) should survival and reproduction be 

 possible, the malformations tend to be transmitted to descendants. The course of ordinary epigenetic 

 development must depend in the main on the nature and characters of the fertilised ovum. 

 Although these are remarkably steadfast, there is evidence that along various lines they are 

 subject to a certain measure of insecurity. Using terms of analogy, we may speak of nodes of 

 instability in the intimate constitution of the germ-cells. The instability is not indeterminate, 

 but tends in each case to open up definite teratological potentialities, and occasionally, from some 

 unknown cause, one of these is followed instead of the normal course of development. The 

 aberration may involve the whole organism as in duplicity or multiplicity, or it may only 

 affect groups of organs or single organs, as in herrnaphroditisrn, pug-head, various fin abnormalities, 

 etc. Monstrosities which arise in this manner may be described as spontaneous or autogenetic. 



II. Most of the recognised teratological types, and particularly the major ones, are also capable 

 of being produced by environmental factors acting during the course of development. Malformations 

 arising in this manner may be termed acquired, and there is no evidence that they are 

 transmissible. It is obvious that the environmental factors in question are not causal in any 

 real sense. They can neither supply motive power nor guidance in the production of any type. 

 All they can do is to cause the course of development to deviate along certain lines which the 

 organism or structure is capable of following. These lines were already marked out through 

 the potentialities referred to under I. The latter are primary to, and indicate the limits of, 

 the former. At the same time, as regards ordinary frequency of occurrence, it is quite possible 

 that, for some types, the instances of acquired may outnumber those of autogenetic abnormality. 



III. On the whole the two groups (namely those of autogenetic and of acquired abnormalities) 

 tend to coincide with one another. At the same time the former will be found to have a 

 wider content than the latter, if induced pathological conditions are left out of count, but here 

 the boundary between teratology and pathology becomes difficult to define. In any case, the 

 fact that a partictdar abnormality appears spontaneously is an argument for, and not against, the 



