EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 301 



differentiating cells, A, consist of a number of Aa, and oi Ab 

 cells? The Aa cells may be supposed to be excluded from the 

 differentiating influence of the environmental conditions and 

 multiply as such. The Ab cells may possess the faculty of de- 

 veloping acidophylic granules and become granulocytoblasts. 

 What will then happen? The Aa cell will divide and give two 

 Aa's, the Ab cell will differentiate into a B cell. At what time, 

 then, does the differentiation of an A a cell into an ^6 cell take 

 place? If between two mitosis, then it is to be expected that under 

 definite conditions at a given time all the Aa cells will differentiate 

 into Ab cells and further into B and C cells (text fig. 1.) Yet the 

 stock of A cells does not show any signs of exhaustion and the 

 hemocytoblasts preserve their existence as such. 



The simultaneous differentiation and proliferation of young 

 cells could hardly be explained otherwise than by a specific 

 process of differentiation during mitosis (text fig. 2.) The 

 division of a cell A must lead to the development of a cell A 

 and a cell A^. The cell A will continue to give rise to cells A 

 and A^ and will truly become the inexhaustible source of the 

 young undifferentiated cells. The cell A^ will undergo further 

 differentiation and will develop finally into a B cell, or a granu- 

 locytoblast, which again will divide into B and BK The cell B^ 

 will differentiate into a C cell, or a granular leucocyte. The 

 differentiation of the hemocytoblasts into granulocytes outside 

 the vessels as well as their development into erythroblasts and 

 erythrocytes within the vessels and their simultaneous inexhausti- 

 ble proliferation could hardly be explained on other grounds. 



I have not been able yet to trace any difference between two 

 daughter cells. However, numerous examples of similar un- 

 equal cell division may be found in the life history of other cells. 

 Boveri represents the differentiation of germ and somatic cells 

 in Ascaris as due to cleavages, which result in formation of cells, 

 of which a part conserves the whole chromatine and another 

 loses a considerable amount of it. Again the differentiation of 

 various cell ranges with persistence of the stem cells in Clepsines 

 depends upon unequal division of the stem cells.*' Conklin 



« This example was kindly given to me by Prof. H. H. Donaldson. 



