28 THE POPULAR SCIENCE MONTHLY 



classes of animals that a detached blastomere may reproduce the whole 

 organism. 



We have still to consider one very interesting kind of tumor, namely, 

 the so-called chorionepithelioma malignum, which also owes its origin to 

 irregularities in the activity of certain embryonic cells but which in 

 some respects differs markedly from the other tumors considered so far. 

 When the segmenting egg attaches itself to the uterine wall it does not 

 only produce the embryo proper, but it gives also rise to certain cells 

 which attach themselves to the uterine wall of the mother, and are 

 mainly concerned in transferring food from the mother to the embryo, 

 thus forming the embryonal placenta, the outer layer of which is called 

 the chorion. These chorionic cells, which are therefore of embryonic 

 origin, migrate already under normal conditions deeply into the uterine 

 wall ; they may even penetrate into maternal blood vessels and be carried 

 to other parts of the body of the mother. Usually these chorionic wander 

 cells perish after some time in the mother, but occasionally they give 

 rise to very malignant tumors which destroy the uterine wall, and form 

 metastases (L. Fraenkel, Marchand). 



Just as these chorionepitheliomata may be produced from the fertil- 

 ized egg cell developing in the uterine wall, so they may occasionally 

 owe their origin to egg cells which develop into embryomata in the 

 germinal glands and especially in the testicle. Here likewise chorio- 

 nepitheliomata may develop. In the structures which I found in the 

 ovaries of guinea-pigs the greater part of the dividing egg cells formed 

 in contact with and probably under the influence of the ovarian tissue 

 of the mother placental tissue and especially migrating cells which 

 penetrated occasionally even into the walls of the neighboring blood 

 vessels. 



We have now analyzed some of the factors concerned in the origin of 

 tumors found in childhood and early adult life. We recognized that 

 they are caused partly by abnormalities of embryonic development, partly 

 by parthenogenetic development of germ cells. At least these are two of 

 the factors concerned in their origin — whatever additional factors may 

 be found in the future. However, the greater number of all tumors, 

 especially the large majority of the typical cancers found in later life, 

 owe their origin to different causes. We can appreciate these causes 

 best, if we consider certain special kinds of cancer which are somehow 

 associated with certain kinds of occupation. 



In general the character of the occupation does not seem to have a 

 marked influence on the incidence of cancer, although it seems that 

 cancer in certain callings (agricultural and forest workers, textile and 

 wood workers, domestic servants) is somewhat more frequent than in 

 others (miners, soldiers, factory workers in general). There are how- 

 ever certain occupations in which a direct connection exists between the 



