ASYIVrMETRICAL DUPLICITY IN THE CHICK. 93 



anterior portion, particularly the medvillary plate region of the 

 embryo : — ■ 



a. The absence of a true head-fold. 



h. ,, ,, J J an anterior amniotic fold. 



G. ,, ,, ,, cranial flexures. 



d. The shortness of the brain resulting mainly from the 



under-development of Telencephalon and Diencephalon. 



e. The condition of the optic vesicles. 



The assumption will therefore be made in the following 

 pages that the primary factor responsible for the abnormal 

 development of Rudiment a was this abnormal arrest of the 

 growth of the anterior portion of the medullary plate, and the 

 attempt will be made to demonstrate how the other structural 

 modifications nre necessary but purely secondary consequences of 

 this initial disturbance of the " causal harmony of development" 

 {Driesch). 



1. The condition of the gut. 



In the first place, we may consider the condition of the fore- 

 gut, particularly with I'eference to the anterior position of the 

 anterior intestinal portal. 



It may be recollected that in the normal chick two main 

 factors are responsible for the formation of the fore-gut : — 



1. A definite forward growth of the brain. 



2. The backward growth of tiie posterior margin of the 



splanchnopleui^ic head-fold owing to pleuro-pericardial 

 coelomic expansion. 



It is generally agreed that the initiation of fore-gut formation 

 is due to the development of the head-fold, which latter itself is 

 due to the more rapid growth of the anterior part of the 

 medullary plate. 



With regard to the precise importance of these two factors 

 there has been considerable difference of opinion, and it may 

 now be accepted as established that the mechanism of fore-gut 

 formation differs considerably in the various groups of verte- 

 brates. With regard to the condition in the chick embryo, 

 Robinson (13) considered that the former factor was that mainly 

 if not solely operative. Other authois (Graper, Rouviere, 4, 14) 

 have contended that there occurs also a backward growth of the 

 anterior intestinal portal, and it may be regarded as established 

 that under normal conditions both these factors play a part in 

 the formation of the fore-gut in the chick (a view which it will 

 be seen is supported by a consideration of this embryo). On the 

 other hand, in the marsupial Dasyurus, Miss Parker (11) has 

 shown that the second factor plays a far more fundamental role. 



Since in the major rudiment of blastoderm E the former factor 

 was largely absent, it is not surprising that a condition of affairs 

 has arisen analogous in many respects with that in Dasyurus, 



