DEVELOPMENT OF THE MAMMALIAN SPLEEN 301 



endothelium of the earhest vessels, as well as from the local mes- 

 enchyme of the blood-forming organs. Lobenhoffer and H. 

 Fischer also claim that the endothelium may assume a hemato- 

 poietic function. 



In 3 to 4-cm. pigs the spleen shows its first signs of blood- 

 cell formation, and is therefore a good object for study. In 

 embryos of this size it is easy to trace all the intermediate stages 

 between the free cells present and the fixed cells of the mesen- 

 chyme. The differentiation of free lymphoid cells at the expense 

 of the mesenchyme does not begin in any definite location within 

 the organ, nor is there any evidence that a change in the vascu- 

 larization of the organ initiates the process, as cells in no 

 relation with vessels and sinuses show signs of differentiation. 

 Cytoplasmic and nuclear changes do not always proceed at a uni- 

 form rate, but, in most cases, the first evidence of differentiation 

 is a slight increase in the basophilic reaction of the cytoplasm of 

 the cell-body (fig. 5). This may begin to appear while the cell 

 is still a part of the general syncytium, or the cell may become 

 entirely free and then assume its more pronounced basophilia. 

 In some cases the nuclear modification precedes that of the cyto- 

 plasm and a typical large lymphocyte nucleus is seen in a cell 

 that is still connected with the mesenchyme. This condition, 

 however, is rare and is usually confined to mesenchymal cells at 

 the margins of the sinuses. 



In making a study of the nuclear structure of the first ameboid 

 cells, one soon finds many intermediate stages between the nuclei 

 of mesenchyme cells and those of the typical large lymphocytes. 

 Figure 5 includes a mesenchyme cell (mes.c.) which shows a nu- 

 cleus that is typical of this tissue. It is a clear, open structure 

 that is round or oval in shape and possesses a rather distinct 

 nuclear membrane. The small amount of chromatin that it 

 contains is distributed in the form of a fine irregular network. 

 Two nucleoli are generally present, but the number is variable. 

 The cell at A shows considerable change in its cytoplasm, but by 

 comparing its nucleus with that of the mesenchyme cell, their 

 identity is at once apparent. In the cell at B the cytoplasmic 

 change is more pronounced and a slight nuclear modification has 



