196 Dr. Bashford and Messrs. Murray and Bowen. [May 30, 



it : (5) Variations in the character of the tumour cells themselves. Any one 

 of these factors may cause a very great deviation from the rate and amount 

 of growth observed previous to the subinoculations introducing it, and 

 invalidate the results of experiments of which information as to possible 

 modification of growth was the object. The variations depending on the first 

 four factors mentioned must be eliminated before variations can be referred 

 to the tumour cells themselves. 



We have taken the following precautions in studying the fluctuations 

 which we believe depend on differences in the tumour cells. 



1. The same race of mice has been used throughout. We have observed 

 differences in the suitability of animals of different colours even among the 

 ordinary English tame mice ; and we have avoided the use of those varieties 

 prized by mouse-fanciers. The wild mouse probably offers more uniform 

 conditions than the tame mouse, but a sufficient stock of uniform age is 

 difficult to obtain, keep and supervise. Jensen's tumour rarely yields a 

 number of successful subinoculations in wild mice equal to that obtained in a 

 control batch of tame mice, and this result when once obtained has not been 

 maintained, but is followed by an increasing difficulty of propagation. The 

 experiments in wild mice may be looked on as control observations to those 

 recorded in tame mice. 



2. The tame mice used have been of uniform age, and from five to seven 

 weeks old. We showed that young animals provide conditions more favourable 

 for the artificial propagation of cancerous tissue than old animals. This 

 conclusion has been amply confirmed by our later experience, and in one of 

 its aspects also by the work of Ehrlich and Apolant,* who state that the age 

 of the animals is without importance and, especially, that old females are not 

 more suitable than young animals for the propagation of mammary tumours. 

 We have found that the greater suitability of young animals is even more 

 marked than we at first suspected. The inoculation of a tumour into young 

 and old animals respectively may occasionally give similar results in the two 

 cases, or even a less favourable result in young mice, still such results are 

 exceptional in our experience. As a rule, a much higher percentage of 

 tumours develops in young animals, and they attain large dimensions in a 

 shorter time after inoculation. The tumours which have developed most 

 rapidly, e.g., attaining a weight of 1*05 grammes in a mouse of 9 grammes, within 

 five days, and those ultimately attaining the largest dimensions as compared 

 with the size of the host, have always occurred in young mice, although 

 tumours of 7 or 8 grammes also develop rapidly in adult animals. Slow 

 growing tumours, which remain of relatively small dimensions, occur both in 



* ' Berl. Klin. Wochenschrift,' No. 28, July, 1905. 



