154 IVAN E. WALLIN 



der into the perivascular spaces and undergo their transformation 

 in these places. The sluggish character of the blood flow in 

 these spaces must account for the retention of the large num- 

 ber of transforming and mature lymphocytes which are present 

 in these situations. The entrance of these cells into the main 

 blood vessels is of a slow nature. 



The tall epithelial cells in the dorsal part of the epipharyngeal 

 ridge also enter into the blood formation. Figure 16 shows a 

 cell taken from the space in the connective tissue of the dorsal 

 part of the epipharyngeal ridge. The nucleus has the morpho- 

 logical characters of the epithelial nuclei. It appears to be in a 

 state of simple fission. The cell in figure 17 was taken from 

 the same locality. Two nuclei are present in this cell which still 

 show the epithelial character. 



Histogenetic comparisons between cells arising from placodes and 



lymphocytes 



In the study of the histogenetic processes in the placodes it 

 was shown by means of various transitional stages that the epi- 

 thelial cells of the placode transform into lymphocyte-like cells. 

 Lymphocytes were shown to develop from the 'epithelial' cells 

 of the gills and gill arches. The lymphocyte-like cells formed 

 from the placodes do not have the same mode of development nor 

 do the transitional forms have the same morphological charac- 

 ters as the lymphocytes and transitional forms developed from 

 the gill and gill arch 'epithelium.' In the placode the nuclei 

 alone migrate away from the original epithelial bed and the 

 transformation occiu*s in the connective tissue meshwork. The 

 complete cell migrates away from the epithelial bed in the gills 

 and gill arches, the transformation occurs in the perivascular 

 spaces and the blood vessels. A small amount of cytoplasm 

 becomes visible in the placode 'lymphocyte' just before it attains 

 its maturity. This cytoplasm stains a gray-blue. The cyto- 

 plasm of the gill and gill arch lymphocytes represent the original 

 cytoplasm of the 'epithelial' cells and stains red. These im- 

 portant differences in the lymphocytes and lymphocyte-like cells 



