THE DEVELOPMENT OF THE VASCULAR SYSTEM. 273 



differentiated from other types of cells; but of the genetic relations between the 

 erythroblasts and the other nucleated cells in the marrow little is known. 



The above description serves only to indicate a general plan of development, and it must 

 be borne in mind that any theory of the life history of the blood cells will meet with opposition 

 at some point. There is one feature, however, that stands out clearly: It is undisputed 

 that the characteristic non-nucleated red blood cells are always derived from nucleated cells. 

 The difficulties and complications which arise in attempting to solve the problem of the 

 genetic relationships among the various types of cells found in connection with developing 

 blood have scarcely been mentioned. 



Erythrocytes 



; Myeloplax 



^^f-~ Fat space 







-"Erythroblasts 

 '~,~' Myelccytes 



j 



'-'' 



Leucocytes Reticulum 



FIG. 250. Section of red bone marrow from rabbit's femur. 



Some investigators entertain the view that the first erythroblasts in the extraembryonic 

 mesoderm arise from mesodermal cells by a peculiar process of nuclear division and subse- 

 quent fission of the cytoplasm. Some of the mesodermal cells become much enlarged and 

 their nuclei divide several times without corresponding divisions of the cytoplasm. The 

 resulting nuclei then escape from the parent cells, carrying with them some of the cytoplasm, 

 and thus form smaller nucleated cells, the erythroblasts. Some of the cytoplasm of the 

 enlarged mesodermal cells liquefies to form the blood serum. 



The question of the origin of blood vessels within the embryonic body has been dis- 

 cussed (p. 237). In connection with this the question also arises as to whether new erythro- 

 blasts may be differentiated form the mesodermal cells along with the differentiation of new 

 vessels, as is the case in the extraembryonic mesoderm. 



