298 BIOLOGY OF THE LABORATORY MOUSE 



the groups that are being compared, seems to have left the problem of (Het in 

 a most unsatisfactory condition. For this reason no attempt is made in this 

 volume to cover the extensive but non-critical bibliography. The whole 

 problem will have to be approached ''from the ground up" by investigators 

 who understand and utilize genetics, biochemistry and mathematics. 



Irritating agents. — There have been several types of experiments dealing 

 with the effects of irritants of various sorts in the response of an animal to 

 implants of tumor tissue. 



Perhaps the simplest approach to this problem is through the introduc- 

 tion of a mechanical irritant which is not able to exert any evident chemical 

 reaction. 



A series of experiments of this sort was reported by E. E. Jones (37) who 

 found that growth of an adenocarcinoma was obtained in a number of mice 

 belonging to stocks, otherwise negative, when a bit of sterile non-dyed 

 flannel was inoculated with a bit of the tumor. 



This interesting result indicated that possibly local factors as well as 

 those affecting general lymphocytic reaction may be operative. 



It would seem that further study of this general held would prove 

 fruitful. 



It is also known that previous exposure of transplantation sites to physi- 

 cal agents such as heat, cold or radiation may affect the percentage of 

 successful implants and their rate of growth. As yet, however, data on these 

 effects are so fragmentary and diffuse as to prevent any general conclusions 

 being drawn. Biochemical irritants of some types have also been used. 



Perhaps a typical and interesting result is that obtained by Koenigsfeld 

 who found that animals painted with carcinogenic tar and inoculated at the 

 same time with a transplantable tumor showed increased response to the 

 former and more rapid growth of the latter. This interesting mutual 

 activation remains unexplained and is in contrast with the experience of 

 investigators who have compared the interaction of centers of benign growth 

 with a center of malignant growth. In this case the usual experience has 

 been that pregnancy slows the rate of growth of transplanted cancer except 

 in the case of certain exceptional tumors. These are mammary adenomas 

 which in some instances have grown more rapidly when the host is pregnant 

 than at other times. 



In all of these experiments, as in those dealing with dietary factors, the 

 present need is for a more accurate control of the too numerous variables 

 which, influencing the fate of the transplant, may serve to mask or to dis- 

 tort the relationship between any one experimental factor and the end result. 



