1904] On the Zoological Distribution, etc., of Cancer. 



67 



vertebrates. The appended List shows the abundance of the material 

 which has thus been examined. The List includes also specimens which 

 we have been privileged to examine through the courtesy of investi- 

 gators abroad, who, subsequently to the inauguration of the Cancer 

 Research Fund, have published descriptions of malignant tumours in 

 the lower vertebrates. It is noteworthy that such growths have been 

 obtained, not only in domestic animals, but also in animals living in a 

 state of nature : wild mouse, codfish, gurnard. 



The clinical, pathological, anatomical, and microscopical characters 

 of these new growths are identical with those found in man in all 

 essential features, although the animals themselves are drawn from the 

 different classes of the vertebrate phylum. A detailed histological 

 description of the various tumours examined will not be attempted 

 here. Only the general significance of the observations in relation to 

 the incidence of cancer in man will be emphasised. 



The great diversity of the habitat, food, and conditions of life 

 generally of the forms in which malignant new growths occur relegates 

 such external conditions to a subsidiary role in determining the inci- 

 dence of the disease, and shows that the essential factors must be 

 sought in the potentialities which reside in the cells constituting the 

 living body. 



The list of specimens, while giving no safe basis of deduction as to 

 the relative incidence of cancer in the different classes of vertebrates, 

 or of the comparative susceptibility of the different sites of the body, 

 is extremely suggestive. 



The large number of epitheliomata obtained in the horse and dog 

 indicates very clearly that malignant new growths are recognised 

 according to the ease with which animals can be examined, and the 

 length of time they are kept under observation. In the same way, 

 numerous malignant new growths have been discovered in the internal 

 organs of cows during the inspection at abattoirs. 



Stated generally, it may be said that malignant new growths are 

 known to occur chiefly in animals which are habitually examined with 

 care, and are unrecorded in forms which are difficult to examine or do 

 not reach old age in considerable numbers. 



The figures are not sufficiently extensive to determine accurately 

 the age incidence of the various types of new growths in different 

 animals, but a relatively higher incidence in old age is apparent. 



The Phenomena of Cell-division in Malignant New Growths. 



The progressive increase in size of malignant tumours is due to the 

 division and increase in size of their constituent cells. The process of 

 cell-division is usually indirect, mitotic division of the nuclei preceding 

 the division of the protoplasm. The protoplasm division is frequently 



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