THE MAST CELLS 



formation throughout the body. With the evolution of the bony fishes the 

 marrow begins to usurp some of the functions of the spleen and becomes the 

 predominant tissue for haemopoiesis. According to Jordan and Speidel (1930, 

 p. 375) the mechanical advantage conferred on the organism by the development 

 of tubular bones also led indirectly to ideal conditions for blood formation. 

 'The diaphyseal content of hollow bones is fundamentally related to the 

 nutritive requirements of the bones. Thus, in a sense, the practical postnatal 

 restriction of hemocytopoiesis to bone marrow in higher forms, especially 

 mammals, was accidental. Arising apparently as an incidental aspect of the 

 mechanism providing for the most favourable nutritive conditions for hollow 

 bones, it becomes appropriated as offering more favourable conditions for 

 blood formation. The spleen, accordingly, becomes of less importance at the 

 higher evolutionary levels after the appearance of long bones. Its original 

 multiple function becomes apportioned among lymph nodes, perhaps thymus, 

 and bone marrow. 1 It is, of course, well known that extensive destruction of 

 red bone marrow even in mammals can be followed by the resumption of extra- 

 medullary blood formation in the original haemopoietic centres of the body, 

 and that after splenectomy the marrow may perform the reciprocal function of 

 producing lymphocytes and monocytes. In view of the multipotent capacity 

 of the marrow for blood formation, it is not surprising that along with the 

 neutrophil and eosinophil leucocytes the marrow also produces variable 

 numbers of blood mast cells. In general there is an inverse numerical relation- 

 ship between the numbers of blood and tissue mast cells in a particular species 

 (Maximow, 1910): however, with few exceptions the number of blood mast cells 

 is small compared with the number of tissue mast cells which continue to be 

 produced at sites of former haemopoietic activity. 



Meanwhile, in course of evolution, another specialized type of tissue has 

 come into being. This is the lymphatic system whose origins can also be traced 

 to the widespread primitive haemopoietic tissue of lower animals. Portions of 

 the multipotent mesenchyme become aggregated into foci specializing in the 

 production of lymphocytes; capsulated lymph nodes make their first appearance 

 in water birds (Further, 1913). Even so, the spleen in birds continues to play 

 a significant role in haemopoiesis, its removal being followed by increased 

 haemopoietic activity elsewhere, as in cold-blooded animals: 'A striking example 

 is presented by the differentiation of the mast cells along the vessels of the 

 omentum. . . . An analogous development of mast cells is seen under the 

 epithelium of the intestine , (Dantschakoff, 1916, p. 485). As the importance 

 of the marrow for haemopoiesis increases still further, the spleen eventually 

 produces little else but lymphocytes and monocytes. Yet there remain, even 

 in the mammals, scattered traces of the original blood-forming tissue which 

 continues to produce the granular cells of the connective tissues. These are 



8 



