92 MR. NOEL TAYLER ON A UNIQUE CASE OF 



B. The nervous system. 



I. Smallness and compression of the anterior two divisions 

 of the brain, the Telencephalon and the Diencephalon. 

 II. Reduction of the optic evagination on the right side. 



C. The gut. 



I. Its shortness, in consequence of the position of the 

 anterior intestinal purtal. 

 II. Absence of an oral plate. 

 III. Thickening and proliferation of entoderm of the gut-roof 



contiguous to the infundibulum. 

 lY. The abnormal prechordal prolongation of the gut. 



D. Heart, vascular system, and coelom. 



I. Ko fusion of heart-tubes. 

 II. Small size of right heart-tube. 



III. Median aortic space beneath the Telencephalon. 



IV. Width of pericardium. 



V. Anterior extension in mid-line of pleuro-pericardial 

 coelom . 



III. Discussion. 



Part A. The Morphogenesis of Embryonal Formation a. 



It would appear advisable, before proceeding to a considera- 

 tion of the ultimate origin of the two embryonal rudiments 

 upon this blastoderm, to discuss in some detnil the morphology 

 of Embiyonal Formation a, the more completely developed of 

 the two rudiments. 



Such a discussion will, I believe, result in fairly definite conclu- 

 sions as to the primary factors responsible for the abnormal devel- 

 opment of Rudiment a, and indirectly to the causes responsible 

 for the presence upon the blastoderm of the second Embiyonal 

 Rudiment (/3). 



There appears to me ample justification for the view that many 

 of the abnoi-malities of Riidiment o, particularly those aiFecting 

 the vascular system and gut, are secondary, in the sense that they 

 are consequent upon an inhibition of the noi-mal development of 

 the extreme anterior portion of the embryo, in particular of the 

 medullary plate region. 



For this abnormal arrest of the groAvth of the anterior region 

 of the medvillary plate the only adequate explanation would 

 appear to be the existence of some germinal defect, which would 

 therefore be the primaiy modification in the sense indicated 

 above. 



That this is indeed the case seems the only view consistent 

 with the occurrence of the followins: abnormalities affecting the 



