THEORIES AND PROBLEMS OF CANCER in 



fact, the authors say that the results were due to the carrying 

 out of a definite plan, using a great number of animals and 

 transplanting as rapidly as possible. <( Our object was to in- 

 crease the malignancy of the tumour cells to the maximum by 

 the continued systematic passage from animal to animal accord- 

 ing to the analogy of bacteriological technique." 2 Whether 

 another interpretation of these results is not more probable will 

 be considered later. 



Bashford, Murray and Bowen 2 have observed alternations or 

 waves in the rate of growth and viability involving several 

 generations of the transplanted tumours with which they have 

 worked and they interpret this as being due to a rhythm in the 

 growth energy. Calkins records similar waves 3 but concludes 

 that they are due to some cause within the cancer cell itself and 

 considers that this cause is probably an intracellular parasite 

 such as Plasmodiophora brassicce. Apart from other considera- 

 tions which make it almost impossible to accept a parasite 

 as the probable cause of cancer, Calkins' paper shows such 

 intrinsic signs of carelessness that the observations described 

 in it cannot be taken as bearing much weight. Another in- 

 terpretation of the significance of these waves of growth will be 

 suggested shortly. 



A general impression conveyed by a consideration of the 

 literature dealing with experiments upon these graftable mouse 

 cancers is that they differ to a large extent from primary 

 malignant growths occurring in the human subject. Metastases 

 or secondary growths are very rare. When they have been 

 described, they have generally followed only upon inoculation 

 with an emulsion of tumour cells and not upon the grafting of a 

 solid piece of tumour tissue. It must be obvious that the 

 former method is one that enables single cells to gain access to 

 a small blood-vessel and be carried to the lungs, where, if they 

 survive, a tumour will develop but only become noticeable later 

 than that formed at the site of inoculation. It is also almost 

 certain that when the emulsion is injected forcibly under the skin 

 or into the peritoneal cavity, isolated cells or groups of two or 



1 It has been demonstrated that a strain of certain disease-producing micro- 

 organisms may be rendered far more virulent by a rapid succession of inocula- 

 tions from animal to animal. 



* Proc. Roy. Soc. 1906, B. lxxviii. 



3 Journ. Exper. Med. vol. x. 3, 1908. 



