ASYMMETRICAL DUPLICITY IN THE CHICK. 83 



9. A Unique C/Use oE Asymmetrical Duplicity {Dupli- 

 citas asymmeiros) \\\ the Clhick. By Noel Tayleh, 

 B.Sc. (Loiid.)"^. (From the Zoological De[)artment, 

 University of London, University College.) 



[Received April 8, 1918 : l^ad May 13, 1919.] 



(Plates I.-llI. t and Text-figures 1 & 2.) 



Index. Page 



I. Inteoductoey Note 83 



II. Desceiptive. The morphology of Blastoderm E 84 



A. General appearance of the Blastoderm and orientation 84 



B. Detailed description of the relation of the two embryonal 



formations and the structure of the lesser (Rudiment 6) 85 



C The morphology of the greater embryonal formation (Rudi- 

 ment a) 86 



D, Summary' of the principal morphological abnormalities of the 



greater embryonal formation (Rudiment «) 91 



III. Discussion 92 



Part A. The morphogenesis of embryonal formation a 92 



1. The condition of the gut 93 



2. The condition of the heart-primordia 94 



3. The anterior port ions of the fore-gut and vascular system ... 96 

 Part B. The origin and relationship of the two embryonal rudiments... 98 



IV. Teeatological Significance AND Conclusions 102 



1. Bearing on the nature of unequal monstrosity, " Duplicitas 



asyinmetros" 102 



2. Conclusions 107 



I. Introductory Note, 



The following paper consists of two main parts, firstly the 

 description of the blastoderm, and secondly a discussion as to its 

 mode of origin and development. Its bearing on teratological 

 problems is also very briefly considered. 



Hitherto little attention has been paid to the abnormal products 

 of morphogenesis, largely, no doubt, owing to the scantiness of our 

 knowledge of the causal factors of development ; nevertheless, it 

 would seem that, in view of the important bearing that the 

 abnormal must have in the interpretation of the norma], it is 

 deserving of more attention than it has hitherto received. It was 

 thought, therefore, that it might be quite worth while to take 

 such a case as the present and subject it to a somewhat detailed 

 consideration from two aspects. Firstly, with respect to the 

 morphogenetic problems which it might itself present, considered 

 in relation to what is known of the factors of normal development, 

 and secondly with respect to its own bearing on our knowledge 

 of the latter and on some of the general problems of teratology. 



* Communicated by Prof. J. P. Hill, F.R.S., F.Z.S. 

 t For explanation of the Plates see p. 109. 



6* 



