ASYMMETRICAL DUPLICITY IX THE CHICK. 107 



however, be again emphasized that it is of extreme importance in 

 the elucidation of the ultimate mode of origin of asymmetrical 

 duplicates. 



2. Conclusions. 



1. Blastoderm E appears unique among hitherto described 

 chick blastoderms exhibiting asymmetrical duplicity, in that 

 both, not merely one, of the embryonal formations exhibit gross 

 morphological defects. 



2. The morphology of Blastoderm E seems only to be explicable 

 on the monozygotic theory of origin, i. e., that both centres origi- 

 nated through some kind of disturbance, the exact nature of 

 which is at pi"esent obscure, from a single and possibly normal 

 germ. 



3. The two centres of embryonal formation, to which the 

 original disturbance gave rise, must have been from their origin, 

 qualitatively and quantitatively, of unlike potential. 



4. The primary modification induced in the greater embryonal 

 formation resulted in the inhibition of the normal growth of the 

 anterior portion of the nervous system and of the formation of 

 the head-fold. 



From this primary modification, indirectly followed various 

 secondary modifications, due to the causal correlation of the 

 various organ -primordia of the embryo. 



5. While in the normal chick both the anterior growth of the 

 medullary plate and the pleuro-pericardial expansion are operative 

 in the formation of the fore-gut, the importance of the latter 

 fa,ctor is indicated by the condition of embryonal formation o, in 

 which, though no true head-fold could have been present, there 

 is nevertheless a well-developed fore-gut. 



6. Blastoderm E appears of extreme interest with reference to 

 the localisation of the " formative material " in the early chick 

 blastoderm. For while it has been experimentally demonstrated 

 that the material of the primitive streak does not enter into the 

 formation of the brain, it appears that, on the other hand, the 

 material from which the anterior region of the medullary plate 

 normally arises may under certain circumstances have the power 

 of giving rise to a primitive streak-like mass of tissue. 



7. Blastoderm E seems to have an important bearing on the 

 problem of the origin and nature of asymmetrical duplicity. 



There has hitherto been ditterence of opinion as to whether 

 the two members of an asymmetrical duplicate could or could 

 not have been originally (to use Wilder's term) "physical 

 duplicates." 



Opinion hitherto has inclined to the view that in the majority 

 of cases this was the case, the inequality of the two components 

 resulting from secondary causes. Authors, however, have recog- 

 nised the difficulty of the problem and the lack of any conclusive 

 evidence. 



