LIFE HISTORY 57 



the sterile alimentary contents of the embryo in utero, 

 and the early development of micro-organisms when 

 food (even, as in this series, the maternal milk) is first 

 taken by the mouth, it would almost seem that it re- 

 quired the stimulus of bacteria in the alimentary canal 

 to provoke the development' of the tissue. 



With regard to the structural form of man's vermi- 

 form appendix we find the same agreement between 

 ontogeny and phylogeny. In the animal series occur 

 types of the globular caecum, the caecum with tapering 

 apex and the caecum with its apex modified into a 

 cylindrical tube of narrow bore sharply demarcated 

 from the rest of the caecum. The human embryonic 

 caecum passes successively through just these stages, 

 first a uniform caecum, then a tapering apex, and lastly, 

 towards the end of fcetal life, a vermiform appendage. 



The asymmetrical position of the adult's appendix is 

 due to a post-natal development of the lateral wall of 

 the rest of the caecum which occurs in early childhood 

 (Keith, 1913). 



Though hardly represented during fcetal life, it is 

 during the early years of childhood that the subepi- 

 thelial lymphoid tissue attains its maximum develop- 

 ment. At this period all the lymphatic glands are 

 relatively large and apparently active. In this con- 

 nection it may be recalled that a lymphocytosis is 

 more readily provoked in early life than in the adult, 

 and that the lymphocytes form an unusually large pro- 

 portion of the white cells in the blood of young 

 children. 



It may be pointed out that the time of greatest 

 activity of the subepithelial lymphatic glands corre- 

 sponds with the period during which the individual is 

 securing immunity against the exanthemata and other 

 infections and it seems that we must correlate those 

 two phenomena — the acquirement of immunity with 



