222 RALPH DOUGALL LILLIE 



Most of these cells have lost the vacuoles of the differentiation 

 stages. The dark brown pigment granules of the primitive blood- 

 cells are still present, but are very few in number. These cells 

 are still found in karyokinesis in the circulation up to 37-mm. 

 body length (and even in the young adult, also, according to 

 Freidsohn, '10), which is as late as I have followed the larval 

 development. 



THE HEMATOPOIETIC TISSUE OF THE MESONEPHROS 



In a stage where the mesonephros is as yet represented only 

 by a single Wolffian duct, larvae of 8- to 9-mm. body length, one 

 sees mesenchyme cells multiplying in the neighborhood of this 

 duct, between it and the aorta, and undergoing a lymphoid 

 metamorphosis (fig. 6). All transitions from branched and 

 spindle-shaped lightly basophil mesenchyme cells (ras) to strongly 

 basophil, rounded, amoeboid lymphoid wander-cells (l.w.c) may 

 be found in a single field of the microscope. In addition, lym- 

 phoid cells with basophil cytoplasm and nuclei in varying degrees 

 of polymorphism are seen in 9-mm. larvae. All transitions from 

 slightly indented to horseshoe-shaped nuclei are found. The 

 more polymorphous of these nuclei show an increase in chromatin 

 and its progressive condensation about the periphery. 



In this stage the first granulocytes appear. An eosinophil cell 

 with about twenty bluish-red (eosin-azure II) granules is shown 

 at ex in figure 6. The first special cells are seen in this stage or 

 slightly later (fig. 7, s.c), and have also been noted in the circu- 

 lating blood, but in very small numbers. 



In a short time, however, the mesonephroic tubules become 

 convoluted and the interstitial tissue increases greatly in amount. 

 Much more granulopoietic differentiation is now seen. At first 

 only eosinophil and special cells appear, but at about 16-mm. body 

 length mast cells and small lymphocytes are likewise found. 

 Since at this stage all the later forms of the blood elements, 

 except spindle-cells, are represented, a description of each and its 

 development may be taken up at this point. 



