THE NATURAL HISTORY OF TUMOURS 561 



in man. When the advance of organisation has rendered 

 reproduction on such a scale as this impossible, the somatic 

 cells still remain able to reproduce portions of the body, such 

 as limbs. This can take place, for example, in many of the 

 cold-blooded vertebrata, and it is possible that it may occur 

 even in man, in the first few days of existence. At least it 

 has been suggested that the little out-growths occasionally 

 found on the sites of limbs that have been lost early in uterine 

 life are attempts at reproduction. But as the progress of 

 development continues, the possibilities of direct reproduction 

 diminish, and when it comes to the most highly organised 

 animals and tissues the limitations are very closely drawn. 

 Reproduction and the formation of tumour buds are both 

 restricted to structures composed of simple tissues. Complex 

 structures such as limbs or organs cannot be reproduced, and, 

 at any rate in adult life, complex tumour buds are never 

 formed. The final stage is reached when specialisation has 

 advanced so far as it has in the cells of the cerebral cortex 

 and of the organs of special sense. These have lost almost all 

 their primitive power. It is very doubtful if they can repro- 

 duce their like or replace what has been injured or give birth 

 to tumour buds. Tumours composed of such highly specialised 

 structures are practically unknown. Development has been 

 carried to such a point in them that they have lost all their 

 other powers in devoting themselves to one. The power of 

 reproducing themselves which has been steadily diminishing 

 in scope as development advanced, is no longer within their 

 capacity, and when it is lost the power of giving birth to 

 tumour buds is lost with it. They are one. 



Tumours from Somatic Cells 



With the exception of such highly specialised structures 

 as these, every organ and every tissue in the body developed 

 from the somatic cells forms its own kind of tumour, just as it 

 possesses its own kind of structure. However much one 

 tumour resembles others in general arrangement, it differs 

 from them just as the parent organ or tissue differs from the 

 rest. Under the term adenoma, for example, are included all 

 tumours built upon the lines of glandular tissue ; but those 

 that grow from the parotid gland are as different from those 



