562 POPULAR SCIENCE MONTHLY. 



I was struck by the fact that keratitis and corneal wounds healed without 

 the appearance of plastic exudation, and I was thus led to study the 

 process of inflammation in other nonvascular structures, such as ar- 

 ticular cartilages and the intima of the larger vessels. In no one of 

 these cases was plastic exudation found, but in all of them were changes 

 in the tissue cells. Turning next to vascular organs, and in particular 

 those which are the common seats of exudation processes, I succeeded 

 in demonstrating that' the presence of cells in inflammatory exudates 

 was not the result of exudation, but of multiplication of preexisting 

 cells. Extending this to the growth in thickness of the long bones — 

 which was ascribed by Duhamel to organization of a nutritious juice 

 exuded by the periosteal vessels — I was thus eventually able to extend 

 the biological doctrine of omnis cellula e cellula to pathological pro- 

 cesses as well; every new formation presupposing a matrix from which 

 its cells arise and the stamp of which they bear. 



Heredity. 



Herein also lies the key to the mystery of heredity. The humoral 

 theory attributed this to the blood, and based the most fantastic ideas 

 upon this hypothesis. We know now that the cells are the factors of 

 the inherited properties, the sources of the germs of new tissues and 

 the motive power of vital action. It must not, however, be supposed 

 that all the problems of heredity have thus been solved. Thus, for 

 instance, a general explanation of theromorphism, or the appearance 

 of variations recalling the lower animals, is still to be found. Each 

 case must be studied on its merit, and an endeavor made to discover 

 whether it arose by atavism or by hereditary transmission of an acquired 

 condition. As to the occurrence of the latter mode of origin, I can 

 express myself positively. Equally difficult is the question of heredi- 

 tary diseases; this is now generally assumed to depend on the trans- 

 mission of a predisposition which is present, though not recognizable, 

 in the earliest cells, being derived from the parental or maternal tissues. 

 But the most elaborately constructed doctrines as to the hereditariness 

 of a given disorder may break down before the discovery of an actual 

 causa viva. A notable example of this is found in the case of leprosy, 

 the transmission of which by inheritance was at one time so firmly 

 believed in that thirty years ago a law was nearly passed in Norway 

 forbidding the marriage of members of leprous families. I myself, 

 however, found that a certain number of cases at any rate did not arise 

 in this way, and my results were confirmed by the discovery of the 

 leprous bacillus by Armauer Hansen. In a moment the hereditary 

 theory of the disease was overthrown and the old view of its acquire- 

 ment by contagion restored. Precisely the same happened a few 

 decades earlier with regard to favus and scabies. Another instructive 



