PATHOLOGICAL STATES IN INVOLUTION. 251 



uterine wall irrevocably by the transformation of the reactive 

 uterine decidua and the chorionic villi and the allantois of the 

 foetus into the combined temporary organ known as the placenta. 

 It may be noted that the non-placental mammals are less 

 exposed to the destructive and toxic efiects of their ofispring 

 as they are born at an earlier stage than in the case of the 

 deciduate mammals. The marsupial foetus is about half an inch 

 in length when transferred to the milk-pouch. It is impossible 

 to look at the placenta without recognizing that it is what we 

 may call a compi'omise growth, one which serves the embryo 

 without destroying the parent hostess. That all mammals are 

 not yet fully armed against any morbid alteration of function 

 in the penetrating chorionic villi is seen, as suggested above, in 

 chorion-epithelioma, where the energy of the villi trophoblasts 

 leads to a malignant overgrowth of the epithelial elements, 

 which the maternal tissues fail to inhibit. The hydatid mole, 

 which does not as a rule become malignant, is a case where such 

 inhibition has been sufficient. All these cases, malignant or 

 benign, support the view held by many that malignancy always 

 depends on the failure of some tissue inhibition, and that if 

 bacilli play any part in the drama it is that of helping to upset 

 the balance between tissues which are fundamentally hostile 

 though they exist normally in symbiosis. These phenomena 

 regarded as a whole establish on a firm foundation the 

 view that the uterus and its reactions during gestation are 

 definite protective processes or variations springing originally 

 from a purely pathological accident in some ancestors of the 

 mammalians. However complex the embryology of the uterus 

 and its appendages, the broad facts are compatible with this 

 view, which is strengthened by the later parasitic history of the 

 offspring after birth. The mammse appear to be a compromise 

 between the needs of the infant and the protection of the 

 mother : they originated in sore or tender spots on the epithelium 

 most exposed to the assaults of the parasite. The growth of the 

 nipple is a complex variation depending on the mechanical action 

 of sucking with a reaction proliferation of the epithelial elements 

 of the sweat and sebaceous glands and an increased blood-supply 

 as special inaternal protections against oral infection. It seems 

 to me that few stronger instances can be found of the fact that 

 the development of many organs, if not all of them, is the result 

 of direct reactions or adaptations which are in the nature of 

 i-epair to tissues otherwise likely to suffer disastrously. 



It is large macroscopic results of this order which enable us to 

 reason about other finer reactions, and even help us to link to the 

 general process those of a microscopic and idtra-microscopic 

 character which we class under " immunity." Such phenomena 

 are reactions under stress which, by the provocation of catalysts, 

 influence life. Much of human character, even, is similar re- 

 action, perfect or imperfect, to the infections to which the race 



