THYMUS-LIKE STRUCTURES IN AMMOCOETES 153 



ranging from 9.5 mm. in length up to the mature individual, 

 blood cells develop from the epithelial cells of the gills, gill 

 arches, and probably the branchial wall by a process of trans- 

 formation. It is the blood formation occurring in the gills and 

 gill arches which is of especial interest in connection with the 

 histogenesis in the placodes. The description of this formation 

 will be limited to the formation of lymphocytes only, in the 31 

 mm. larva. 



The similarity of the cytoplasm of the gill epithelium to the 

 cytoplasm of some of the blood cells was early noticed. This 

 similarity was found to be due to an actual relationship between 

 the two kinds of cells and thus not a mere coincidence. This 

 relationship was demonstrated when epithelial cells were found 

 migrating through the walls into the lumen of the blood channels 

 in the gill. Figure 13 represents a part of the gill epithelium 

 and a blood vessel and shows an epithelial cell beginning its 

 migration into the vessel. Figure 12 a shows another epithelial 

 cell in the state of migration, almost half of the cell in this case 

 is inside of the vessel. The cells to the left in figure 12 repre- 

 sent blood cells (in the vessel) in various stages of transformation. 

 In this figure, the chief stages in the transformation of the epi- 

 thelial cells to lymphocytes are represented. The lettering a 

 to h in the figure shows the hne of transition from an epithelial 

 to the mature lymphocyte. 



In this formation of lymphocytes, it is noteworthy that the 

 entire epithelial cell migrates from the 'epithelium' and takes 

 part in the transformation. The transformation consists of a 

 reduction in the size of the nucleus and also in the amount of 

 cytoplasm. The cytoplasm retains its staining qualities through 

 these changes so that even in the mature lymphocyte a cytoplas- 

 mic ring which stains red may be seen in many instances. It is 

 very seldom that a lymphocyte containing a cytoplasm which 

 stains a blue or gray-blue is. seen in these situations. All the 

 transforming cells have a cytoplasm which stains red with the 

 haematoxylin-eosin stain. 



Although some of the epithelial cells in this transformation 

 migrate directly into the blood vessels, the great majority wan- 



