286 ON GROWTH AND OVERGROWTH 



the syncytial cells of the placenta ; they are, as many 

 have long held, of foetal origin, derived from the 

 outer layer of the foetal epiblast. They have, physio- 

 logically, well-marked powers of eroding or breaking 

 down the uterine tissue, and through their agency it 

 is that the villi penetrate into the maternal blood 

 sinuses. Physiologically, that is, they possess what 

 we regard as malignant properties. The highly 

 malignant tumour, formed as a result of their over- 

 growth, the so-called deciduoma or syncytioma 

 malignum, is thus clearly an example of cells which 

 are the product of one individual invading the tissues 

 of another individual. Placental moles belong also 

 to this class. 



B. BLASTOMATA 



Tumours composed of the products of aberrant growth of celh 

 and tissues of the individual in whom they develop. 



I. In which abnormal cell relationship precedes the neo- 

 plasia by a definite interval, and apparently predisposes to the 

 aberrant purposeless growth. These abnormal relationships may 

 be (a) inherited, (6) acquired, and then either of ante-natal or 

 post-natal acquirement. 



(i.) Heterochronic. Tumours in which the primary abnormal 

 cell relationship is due to the persistence of tissues 

 from a previous stage of development, which tissues 

 normally undergo atrophy and disappear. Examples : 

 Tumours developing from the thyroglossal duct, 

 Gartner's duct, branchial clefts, etc. 



(ii.) Heterotopic. In which the abnormal cell relationship 

 is due to the displacement of cells and tissues during 

 the process of growth, so that the tumours develop 

 from " cell rests." Examples : Cancer of accessory 

 thyroids and other tumours developing in accessory 

 glands, rhabdomyomata, 1 and suprarenal tumours of 

 the kidney, etc. 



(iii.) Heterotrophic. Tumours in which the abnormal cell 

 relationship is due to unequal growth of different 

 1 [Tumours composed of striated muscle elements.] 



