THE TRUE C^CAL APEX, OK THE VERMIFORM APPENDIX. 85 



appendix of the rabbit as to constitute its most marked and 

 prominent feature. 



The appearances presented in a transverse section of a rabbit's 

 appendix are at first difficult to explain, but are elucidated by 

 a study of its development. 



In a full-term fa3tal rabbit (see PI. VII. 2) the mucous mem- 

 brane consists of villi projecting into the lumen of the gut, and 

 covered by transparent columnar cells. The submucous coat 

 is but ill defined, and contains little lymphoid tissue. A few 

 days after birth, however, the amount of lymphoid tissue in the 

 submucosa becomes much more marked, and tends to grow 

 inwards towards the lumen of the gut, and between the widely 

 separated villi of the mucous membrane. Concurrently with 

 this increase in the amount of the lymphoid tissue in the sub- 

 mucosa, the free extremities of the villi become slightly flattened 

 and bulbous. 



In rabbits about one week old the changes in the microscopical 

 appearances of the appendix are very marked. The submucosa 

 has become much thickened, and now constitutes the most 

 prominent of the four coats; this thickening is entirely due to 

 an increase in the amount of lymphoid tissue, which grows in 

 the direction of least resistance — that is, inwards towards the 

 lumen of the gut, and between the villi of the mucosa, extending 

 as far as the apices of the villi. 



Such a study of the development of the appendix of the rabbit 

 not only explains the features of the appendix of the adult 

 rabbit, but also establishes the very important fact that at birth 

 there is but little lymphoid tissue in the rabbit's appendix, and 

 that within a week after birth the lymphoid tissue has increased 

 to such an extent as to convert the rabbit's appendix into a 

 lymph gland. The developmental history also shows, that though 

 the flattened apices of the villi in the appendix of the adult 

 animal may appear to touch each other, they in reality never 

 do so, consequently there is nothing to prevent the leucocytes 

 contained in such enormous amounts in the submucosa of the 

 appendix from being discharged into the lumen of the gut. 



The facts to be gleaned, then, from a study of the structure 

 and development of the appendix vermiformis of the rabbit are, 

 that the characteristic feature of the appendix in the rabbit is 



