104 MR. NOEL TAYLER ON A UNIQUE CASE OF 



With regard to the origin of unequal duplicates, most writers 

 liitherto seem to have inclined to the opinion that they were 

 originally physical duplicates, the asymmetry resulting from 

 secondaiy deformation. Thus Wilder in his earlier paper (18) 

 alvanced this opinion, basing it partly on the fact that where it 

 is possible to determine the sex of the " parasite," " it seems 

 al ways to be the same as that of the autosite." In his later paper 

 he seems to have modified somewhat his earlier views, " all are 

 not necessarily deformed." 



The discussion may no doubt become somewhat pedantic unless 

 it be constantly borne in mind that the numerous morphological 

 types of unepial duplicates must almost certainly differ con- 

 siderably in their ultimate origin. " Fiir jede Forme, eine 

 Spezieluntersnchung eintreten " (Schwalbe). 



On the other hand, were we in a position to establish a classifi- 

 c ition of unetpial monstrosity based upon their morphogenesis, 

 tlie most fundamental distinction that could possibly be made 

 would almost certainly be that between those which did and those 

 which did not arise from like centres, assuming for purposes of 

 argument that both of these types do in fact occur. 



Bearing in mind that the present discussion is concerned with 

 the avian blastoderm, we may venture, then, to postulate three 

 fundamental problems raised by a consideration of unequal 

 monstrosity. 



1. Can the two embryonal centres of an asymmetrical duplicate 

 be explained as arising ultimately from a single, normal centre ? 

 In other words, is their nature dizygotic or monozygotic ? 



2. At their time of oingin are the two centres like or unlike ? 

 8. In the latter case — 



(a) What are the causal factors responsible for the dissimilax'ity 



of the centres ? 



and 



(b) What is the relation between the dissimilarity of the 



centres and the production of the two centimes from an 

 originally single centre ? 



In other words, is there any direct causal connection 

 between duplicity and asymmetry ? 



Now, clearly, could a definite answer be given to these questions 

 m every well defined case of asymmetrical duplicity that is pl.oced 

 upon record, we should be far advanced towards a comprehension 

 of the main types of these, and of their morphogenesis. 



The answer to these questions, as far as the blastoderm described 

 in this paper is concerned, seems fairly definite, if the conclusion 

 arrived at in section B siipra be valid. There it was concluded : 



1 . That the blastoderm was certainly monozygotic. 



