BLOOD 15 



green. In the marrow it is possible to find reticular cells in the 

 cytoplasm of which a few dark bodies are present ; these are 

 certainly not typical mitochondria, although they may well be pre- 

 cursors of such structures. It is to the method of depleting the 

 bone marrow of pigeons by starvation that we owe the discovery 

 of this cell-type. The next more mature cell discoverable con- 

 tains definite rod-shaped mitochondria that stain with Janus 

 green, the subsequent stage being an element in which the 

 mitochondria are in the form of quite short rods, present in con- 

 siderable numbers. Up to this developmental condition there arc 

 no structures staining with neutral red, and it is highly probable 

 that these three types of cell are not distinguishable from one 

 another by the ordinary methods of staining used in clinical 

 ha^matology : all these forms would be termed myeloblasts by the 

 polyphyletists, whereas the monophyletists would term them 

 lymphoidocytes. The next phase of maturation is the appearance 

 of two or three small granules staining with neutral cell : this is 

 the so-called myelocyte, type A, which must correspond with the 

 premyelocyte of clinical hsematology. There is gradual increase 

 of neutral red granules as the cells become more mature, and the 

 mitochondria get fewer and fewer, until in myelocyte, type C, they 

 are almost completely replaced by neutral red granules. At this 

 stage the nucleus has become indented, and we are dealing with 

 the metamvelocvte of ordinarv terminologv ; that is, an almost 

 mature granulocyte is characterised by practically complete 

 absence of mitochondrial structures staining with Janus green. 



Granular Leucocytes. Only two aspects of the extensive 

 subject of the granular leucocytes need be discussed. First, the 

 teaching of Arneth which is not recent, although its ramifications 

 are of very modern date. Briefly, Arneth contends that the 

 greater the degree of complexity of the nucleus of a granular 

 leucocyte, the greater is its stage of maturity. It is obvious that 

 a myelocyte with a completely round nucleus is younger than a 

 metamyelocyte with a slightly indented one, but it is not quite so 

 certain that a polymorphonuclear cell with two lobes in the 

 nucleus is necessarily younger than is one witli three or more lobes. 

 Nevertheless most workers who have spent much time on this 



