98 CHARLES R. STOCKARD 



Finally, in the mammalian embryo, one organ after another 

 ceases to offer the necessary harbor for dividing blood cells until 

 the red bone marrow is the only tissue presenting the proper 

 relationship of spaces and vessels, and here alone the erythro- 

 poetic function exists to supply the red blood cells for the entire 

 body circulation. The red blood corpuscles are always produced 

 so as to be delivered into the vessels and thus very soon occupy 

 an intra-vascular position, while the white blood cells arise and 

 remain for some time among the mesenchymal tissue cells in 

 an extra-vascular position. 



11. Lymphocytes and leucocytes along with the invertebrate 

 amoebocytes are all generalized more or less primitive wander- 

 ing cells, and are almost universally distributed throughout the 

 metazoa. 



Erythrocytes are very highly specialized cells with a peculiar 

 oxygen carrying function due to their haemoglobin content. 

 In contrast to the universal distribution of the leucocytes the 

 erythrocytes are only found in the- vertebrate phylum, except for 

 a few cases existing in some of the higher invertebrate groups. 

 Yet even in these particular cases the oxygen carrying blood 

 cell never presents the typically uniform appearance of the verte- 

 brate erythrocyte. The oxygen carrying function in inverte- 

 brates is usually confined to the liquid plasma. 



Typical vascular- endothelium is widely distributed in the 

 animal kingdom and appears to be formed from a simple slightly 

 modified mesenchymal cell. 



These three very different types of cells all seem to arise from 

 mesoderm the mesenchyme. Yet the present investigation 

 would indicate that each arises from a distinctly different mesen- 

 chymal anlage. 



The erythrocyte anlage is localized and perfectly consistent 

 in the quality of its production. 



The lymphocyte and leucocyte anlage is more diffusely ar- 

 ranged and not definitely localized in any particular cell group. 



The vascular* endothelium appears to be formed in loco in 

 almost all parts of the embryonic, body, and its formation is 

 absolutely independent of a circulating fluid or the presence of 

 blood cells. 



