IMMUNITY 9 



the centrosome being inactive, except, perhaps, in amitosis of spermatids 

 and some leucocytes. At present the most reasonable view of amitosis 

 is that cells dividing in this way are in process of degeneration and are 

 approaching the end of division of any kind. 



EHRLICH'S SIDE-CHAIN HYPOTHESIS 



This hypothesis is supposed by some physiologists to account for the 

 processes involved in the nutrition and multiplication of animal cells. 

 It may be diagrammatically illustrated by supposing that the cell sends 

 out from its surface little prominences in the form of a side chain. 

 These hypothetical prominences are called receptors, although their 



Prote/cf 



Molecule 



Pi?- 7- Animal cell with three receptors, one each for carbohydrates, proteids and fats. 



form has been inferred rather than seen. The different receptors 

 have affinities for special nutrient matters; for example, there are cer- 

 tain receptors that have an affinity for proteid molecules, others for 

 carbohydrates, and others for fats. When the affinity of a special 

 receptor is satisfied by the appropriation of the special molecule with 

 which it is combined, a new receptor is sent out, with the same affinity, 

 and so on until the nutritive requirements of the cell are satisfied. The 

 same process goes on between the carbohydrate molecules and the 

 fat molecules and their appropriate receptors ; and thus the cell is 

 nourished. 



IMMUNITY 



The side-chain hypothesis leads naturally to a theory of the de- 

 struction of cells by the action of toxins ; and this, to a theory of 



