ASYMMETRICAL DUPLICITY IN THE CHICK. 97 



Now, on the assumption that iu Rudiment o the dorsal 

 thickening of the gut-wall in the infundibular region represents 

 the protochordal plate, interesting deductions follow as to the 

 morphological relationships of the associated parts. In the first 

 place, it follows that that portion of the thickened gut-roof 

 immediately in front of the point where the latter makes contact 

 with the infundibular region of the Diencephalon must be 

 regarded as the equivalent of the entodermal moiety of tlie oral 

 plate of the normal chick : and further, that portion of the 

 thickened entoderm immediately po^teiioily to this as the mor- 

 phological antei'ior limit of the gut, i. e., the prechordal gut 

 (Seessel's pocket) of the normal chick (PI. I. fig. 7 A). 



If this interpretation be correct, we must conclude that the 

 singukar anterior prolongation of the gut, which is well shown 

 both in the transveise and longitudinal sections (fig. 4, E 2, and 

 fig. 7 B, Ant.eoct.F.G.), is morphologically {i. e., in reference to 

 the normal condition) a part of ventral gut entodei'm, and that 

 that j)ortion of the gut-roof which extends from the point of con- 

 tact with the infundibulum to its topograj^hical anterior end Z 

 (fig. 7 B), and which is therefore apparently dorsal, is morpho- 

 logically ventral, being the equivalent of that part of the gut- 

 floor of the normal embryo which extends from the posterior 

 limit of the oral plate to, say, the point Z' (fig. 7 A). 



The dispositions of the anterior portions of the aortic system 

 can only, it seems to me, be consistently interpieted on this 

 view. The median vessel between the telencephalon and the 

 anterior prolongation of the gut (fig. 4, E 2, Ant.aor.sj).) is 

 formed by the union of the continuations of the two dorsal 

 aortte which pass round the infundibular-gut connection ; ventro- 

 laterally it is continuous with the anterior continuations of the 

 two lateral heart tubes. 



Now there seems to be no room for doubt that this median 

 vessel X (fig. 7 B) must be regarded as morphologically the 

 median ventral aorta X' (fig. 7 A), the portions of dorsal aortse 

 which pass round the infundibular-gut connection to open into 

 it represent the mandibular arches, wliile a most significant fact 

 is the absence of any fusion between the anterior continuations 

 of the two heart-tubes on the apparent or topographical ventral 

 side of the gut. 



The portion of the gut-wall with which this median aortic 

 space is in immediate relation, though apparently dorsal, corre- 

 sponds, it must be recollected, to that part of the entoderm 

 which forms a part of the floor of the gut of the normal chick. 



These modifications of the normal morphology of the head no 

 doubt result from the absence of the normal growth zones of the 

 anterior end of the medullary plate and brain. 



1. Primarily, the absence of normal head-fold formation 

 inhibited the possibility of any conjunction of the entoderm of 

 the fore-gut with ventral head-ectoderm to form an oral plate. 



2. The presence of a considerable part of what should normally 

 Piioc. ZooL. Soc— 1919, No. VII. 7 



