Cancer. 



403 



from which the tumor took its orig-in ; in a greater or less measure 

 the cancer cells preserve the peculiarities of the epithelium of the 

 place of inception, -as a tendency to kcratinization, to formation of 

 mucin and other secretory substances. This retention of original 

 characteristics, which is well seen in the metastatic nodes, often 

 makes it possible to determine from microscopic examination the 

 primary source or origin of a cancer. This is, however, often sub- 

 ject to modification. The growth of epithelial cells in unfamiliar 

 positions, their separation from their normal substructures, must 

 directly, because of the altered nutritional conditions and the vari- 

 ous factors of tissue resistance, occasion variation in their 

 shape and size. In consequence we often ^neet with morphological 

 variations and anomalies which differ so widely from the original 



Fig. 120. 



Cancer of glans penis of horse. 



characteristics of the mother cells, that it may be quite impossible 

 to come to any conclusion as to the source and type of the tumor 

 from the morphology of its individual cells. In addition to this 

 polymorphism of the cancer cells there may also occur one or other 

 of a variety of retrograde metamorphoses in the luxuriantly growl- 

 ing and irregularly nourished cells, rendering them completely un- 

 recognizable ; so that at times the real character of the cellular ele- 

 ments can be made out only in the more recent parts of the tumor. 



Corresponding to the major types of cutaneous, mucous mem- 

 brane and glandular epithelium, three classes of cancers may with 

 Diirk be recognized : 



I — Surface cell cancers of the skin and mucous nuvnhranes cov- 

 ered -with flat epithelium (called brietly by most authors squamous 

 epitheliomafa.) 



