210 RALPH DOUGALL LILLrE 



subject of the present paper : a reexamination of the early devel- 

 opment of the blood elements in Bufo, and for this purpose the 

 California species B. halophilus Baird and Girard was found to 

 be the most convenient. 



Bryce ('04) derives the blood and endothelium from the 

 splanchnic mesenchyme. The yolk-laden primitive blood-cells 

 quickly differentiate into a larger type with peripheral fibrils: 

 the erythroblasts, and a smaller lymphoid form. The larger type 

 pass over into yolk-free, oval, flattened cells in which hemoglobin 

 appears. The loss of the yolk may precede the appearance of 

 hemoglobin by a short time. In the next phase erythropoiesis 

 continues, the stem cell now being a large mononuclear cell. ' The 

 mesenchyme gives rise to large mononuclear cells, polymorpho- 

 nuclear leucocytes, and granulocytes. The splanchnic mesen- 

 chyme, the intrahepatic mesenchyme, and the interstitial mesen- 

 chyme of the pro- and mesonephros are the chief sites of blood 

 formation. The last, at least, sets free leucocytes into the blood, 

 as they are more numerous in the cardinal vein than in the aorta. 

 Likewise, small lymphocytes appear. With the development of 

 the spleen further hematopoiesis is confined to it, to the lympho- 

 renal tissue, and to the gut-wall mesenchyme. The splenic 

 trabeculum cells give rise to erythrocytes through large lympho- 

 cytes, also to lympholeucocytes, finely and coarsely granular 

 eosinophils, and basophil granulocytes. In the later stages the 

 lymphorenal tissue is almost entirely lympholeucopoietic. Leuco- 

 and granulopoiesis are strictly extravascular, erythopoiesis intra- 

 vascular. This last conclusion is reached also by Bizzozero 

 ('90) in birds, Danchakoff ('08 a, '16 a) in birds and reptiles, 

 Jordan and Flippin ('13) in reptiles, and Maximow ('09 a, '10) in 

 mammals, selachians, and amphibians. And when erythrocytes 

 are formed outside of the vessels they soon degenerate, while 

 granulocytes are only formed intravascularly where the circula- 

 tion has been stopped up (Danchakoff, '08 b). 



In the main the differentiation of the blood-cells in the other 

 vertebrates follows the outline given by Bryce for Lepidosiren 

 (Maximow, '09 a, '10; Danchakoff, '08 a, '16 a; Mietens, '10; 

 Jordan and Flippin, '13). 



