BLOOD AND BONE-MARROW OF FROG 471 
which blood-platelets arise from megakaryocytes in mammalian 
marrow thus appear to be coincidences of these two cytoplasmic 
properties of active and degenerating leucocytes respectively. 
This conclusion is further strengthened by the fact that the hemo- 
genic giant-cells of the yolk-sac (e.g., 12-mm. pig embryo) like- 
wise produce typical platelets in abundance." 
SUMMARY 
1. The blood of the leopard frog contains the following cellular 
elements; erythrocytes, thrombocytes, large and small lympho- 
cytes, and neutrophilic, eosinophilic, and basophilic granulocytes. 
2. Thrombocytes, neutrophilic granulocytes, and lympho- 
cytes contain a variable quantity of metachromatic granules. 
Both thrombocytes and lymphocytes show conspicuous granular 
pseudopods which may constrict to form platelet-like bodies. 
3. The polymorphonuclear neutrophilic leucocyte of the frog 
resembles the cell of this designation in certain mammalian bloods. 
It is characterized especially by its conspicuous astral system, 
its extremely lobulated nucleus, and the presence of metachro- 
matic granules arranged in lines radiating from the centrosphere. 
4. The red bone-marrow contains the following types of 
myelocytes; erythroblasts and thromboblasts, only intravas- 
cularly: lymphoblasts, both intravascularly and extravascularly ; 
granular myelocytes, including neutrophilic, eosinophilic, and 
basophilic cells. All of the leucocytic series show pseudopods, 
which may constrict to form ‘hyaline’ bodies and granular 
platelet-like corpuscles. Certain lymphocytes differentiate also 
into plasma-cells and giant-cells extravascularly. 
5. All the types of myelocytes can be traced back to a similar, 
apparently identical, progenitor, a lymphocyte-like cell or hemo- 
blast arising from the mesenchyma. The evidence from this 
material is wholly in accord with the monophyletic theory of 
blood-cell origin. Environmental conditions are apparently the 
chief factors which determine the line of differentiation a certain 
hemoblast shall take. Cords of hemoblasts enclosed by endo- 
thelium produce erythroblasts and thromboblasts, or persist in 
