THE TRUE CiECAL APEX, QR THE VERMIFOllM APPENDIX. 97 



vertebrates, it is collected into a specially differentiated portion 

 of the intestinal canal — the vermiform appendix. 



This tendency for the lymphoid tissue to be specially collected 

 together into a differentiated portion of the alimentary canal 

 becomes more marked in certain groups of the vertebral series 

 as follows : — 



In the pigeon, and in all the other birds examined, the lym- 

 phoid tissue occupies the whole length of the csecal wall. 



In two of the rodents, the mouse and the rat, there is an 

 attempt at a constriction in the caecum, and the lymphoid 

 tissue is confined to its apical portion. 



In the rabbit this constriction has become much more clearly 

 defined, there being both a cfecum and an appendage ; the 

 appendage contains such large quantities of lymphoid tissue as 

 to be nothing but a lymph gland, whilst the csecum contains 

 but little lymphoid tissue. 



In the higher Anthropoids, the Simiidge, there is a vermiform 

 appendix. 



Lastly comes Man, with a csecum containing little lymphoid 

 tissue, and a vermiform appendix, with large quantities of such 

 tissue. 



Assuming this view to be correct, that the lymphoid tissue 

 tends to be collected together into a specially differentiated 

 portion of the intestinal canal, a knowledge of evolution would 

 almost inevitably require the occasional occurrence of an appen- 

 dix in some one of the lower animals. This actually is the case,, 

 not in one but in at least two of the lower animals — the 

 wombat and the capybara. 



Conclusions. 



1. Lymphoid tissue is the characteristic feature of the caecal 

 apex. The vermiform appendix of Man is therefore represented 

 in the vertebrate kingdom by a mass of lymphoid tissue, situated 

 most frequently at the c^ecal apex. 



2. As the vertebral scale is ascended, this lymphoid tissue 

 tends to be collected together into a specially differentiated 

 portion of the intestinal canal — the vermiform appendix. 



VOL. XXXV. (N.S. vol. XV.) — OCT. 1900. G 



