190 JOHN STEPHENS LATTA 



Typical erythropoiesis, however, has been observed by many 

 authors in the mesenchyme or embryonic connective tissue in 

 various places in the embryonic body. Saxer ('96) found it 

 occurring in the mesenchyme in and about the musculature of 

 the neck, in subcutaneous tissue, and various other places. 

 Maximow found in rabbit embryos, it might be found in practi- 

 cally any place in the body mesenchyme, especially of the head 

 region. Badertscher ('15), in study of pig embryos, found 

 developing red blood cells in portions of the neck and upper 

 thorax, as well as in the cortex and medulla, and interlobular 

 septa of the thymus. Danchakoff also described erythropoiesis 

 in the mesenchyme of the head in younger embryos, and in 

 older stages also in the mesenchyme in other parts of the body. 



Erythropoiesis, as occurring in these places, is mainly extra- 

 vascular, although Danchakoff found it occurring both intra- 

 and extravascularly. This, however, she does not consider as 

 true, typical erythropoiesis for few or none of the thus formed 

 erythrocytes get into the blood stream. 



Aside from erythropoiesis occurring in the body mesenchyme, 

 it is well known that it occurrs in, or at least among, the mesen- 

 chymal or reticular cells in the embryonic liver and also in the 

 embryonic spleen to some extent (in a few animals also in the adult 

 spleen). 



But the process of erythropoiesis, according to our present 

 knowledge, is in postfetal life, limited to the red bone-marrow, 

 with the exception of the first few days after birth, before the 

 erythropoietic activity of the liver and of the spleen has ceased, 

 and of the spleen, which may under pathological conditions 

 (and in some animals, as the skunk, normally) reassume or con- 

 tinue its embryonic erythropoietic activity. 



With this fact in mind, that, normally, after the second week 

 of postfetal life, the sole seat of erythropoiesis known is the 

 red bone-marrow (except in the spleen as stated), it is interest- 

 ing to observe that, in studying sections from the appendix and 

 the iliac tonsil of rabbits of an age varying from two to six weeks 

 postpartum, there may be found in the connective tissue of the 

 submucosa of these regions groups of cells in different stages of 



