22 TISSUES AND ORGANS 



in persons in whom no immature blood cells can be found in the 

 circulation, that is, no source for the multi-potent cells can be 

 discovered. 



At this stage we are inevitably faced with the problem of the 

 nature of the cell now usually termed a monocyte. This element, 

 which forms about 6 per cent, of the leucocytes in normal blood, 

 is still the subject of lively controversy. An origin from myeloid 

 tissue is supported by the users of the supra-vital method of 

 staining. Thus it is stated that the very yovmg monocyte, the 

 monoblast, is quite distinct from the myeloblast. It contains 

 relatively few small mitochondria which lie in the indentation of 

 the immature nucleus. In the mature form mitochondria are 

 much more numerous and are less sharply localised in position ; 

 furthermore, a few neutral red granules appear, usually in the 

 nuclear indentation around the centrosome. There is, however, a 

 totally different view, which is that the great majority of mono- 

 cytes arise somewhere in the circulation from multi-potent 

 haemocytoblasts. This is not quite the same as the contention of 

 the trialistic school which holds that monocytes are derived from 

 a third hgematopoietic tissue, the reticulo-endothclium itself ; 

 rather, the attitude is that the monocyte is a peculiar manifestation 

 of the ubiquitous " lymphoid " cell beloved of the monophyletists. 



The last point in connection with the blood that requires to be 

 mentioned at present is the peculiar specificity of arrangement of 

 the reticulum in the normal blood-forming tissues. For example, 

 the reticulum in the bone-marrow has a special relationship to the 

 blood vessels from which it radiates after having formed an 

 incomplete investment. In spite of this peculiarly typical 

 arrangement there is enormous lability of structure, and innumer- 

 able stimuli that affect the activity, and therefore the structure of 

 the bone-marrow results in a rearrangement of the reticulum. 



PITUITARY 



Recent advances in intra-cranial surgery have made the pituitary 

 gland a subject of great interest and practical importance, so that 

 we are quite justified in devoting some space to reviewing new 

 observations on its cellular composition. First, it must be made 



