ASYMMETRICAL DUPLICITY IN THE CHICK. 103 



meine a un parasite, par consequent incapable de vivre par lui- 

 meme, et se nourissant aux depens dii premier dont il n'est 

 physiologiquement qu'un simple appendice." 



According to Wilder (18) : " Tliese monsters consist of two 

 components of ver}^ unequal development, the one (auto.site) 

 being normal or nearly so, and the other (pai-asite) quite incom- 

 plete and attached to the first as a dependent growth usually 

 adhering to some point on the ventral side." 



Schwalbe (16) defi.nes them as: " Die asymmetrischen Doppel- 

 bildungen, die zweite grosse Hauptgruppe der Doppelmissbildun- 

 gen, sind dadurch characterisiert, dass ein Individualteil eiiie 

 bedeuteud geringere Ausbildung aufweist (Parasite) als die 

 andere, vollig ausgebildete (Autosite)." 



Kaestner (6, 7) more briefly speaks of them as, " Doppel- 

 bildungen mit unsymmetrisch gelegenen oder ungleichmausig 

 entwickelten Komponenten." 



It is instructive to compare these definitions : the latter three, 

 those of Wilder, Schwalbe, and Kaestner, it will be noted, are 

 based purely on morphological data, in striking contrast to the 

 original definition of St. Hilaire, which was based also on a 

 physiological conception, that of nutrition. This latter is now 

 clearly inadmissible, and in any case was on]y reiidered possible 

 by the lack of early stages. The only strictly valid definition 

 and classification of unequal monstrosity would be based not on 

 physiological, or even on morphological, but on morphogenetic 

 data, in other words on ontogeny, just as there is oidy one natural 

 classification of animals, that based on phylogeny. 



In other words, the only rational or natural classification must 

 be based on a consideration of past history, and through that of 

 causality. 



Now the fundamental principles which are raised by a con- 

 sideration of the morphogenesis of asymmetrical duplicity appear 

 to be first, the relationship of the two centres at their time of 

 origin, and secondly, the possibility and extent of a subsequent 

 modification of these relations due to secondary causes, which may 

 or may not act in such a way as to obscure the original condition. 

 Obviously the rational classification of double monstrosity will 

 only be founded on the fullest possible enlightenment as to the 

 former of these questions, which in turn can only follow from a 

 knowledge as to the extent and nature of secondary modification. 

 The essential problem was appi-eciated by Wilder in the passage 

 quoted above, " this form of monster has never been studied for 

 the purpose of testing whether or not the components were ever 

 originally physical duplicates." 



The question is, after all, merely an aspect of a wider one which 

 has been stated as the " ontogenetic permanency of teratological 

 organisation." With reference to this, Stockard (17), speaking of 

 the Cyclopean condition, says : " the cyclopean defect is present 

 from the first in the same condition as it will continue throughout 

 development." 



