HEMAPOIESIS 



215 



The yolk-sac wall is the earliest locus of blood-cell origin. This struc- 

 ture is a vesicle lined by columnar entodermal cells, invested by mesen- 

 chyme, the surface cells of which become arranged in the form of a mem- 

 brane, the mesothelium. The first anlage of blood appears in the shape 

 of irregular masses and cords of cells next the entoderm. These cords 

 anastomose at the same time that they acquire lumina through a process 

 which involves the peripheral 

 differentiation of cells into 

 endothelium and the appear- 

 ance of plasma in which the 

 central cells are suspended. 

 The very intimate spatial re- 

 lationship of these blood- 

 islands to the entoderm has 

 led many to incline to an en- 

 todermal origin of the earliest 

 blood anlages. The layer in 

 which blood first appears in 

 the yolk-sac has been named 

 angioblast, a term which does 

 not commit one to any view 

 as to primitive ger'm layer 

 origin. However, the impor- 

 tance of the matter of origin 

 seems great only when one 

 believes in the strict specifi- 



FIG. 229. WALL OF YOLK-SAC OF A 13 MM. 

 HUMAN EMBRYO (FiG. 210), SHOWING A 

 SMALL BLOOD ISLAND (B. I.) AND SEVERAL 

 SMALL BLOOD VESSELS CONTAINING ERY- 



THROCYTES. 



The wall consists of stratified columnar and 

 cuboidal entodermal cells (to the right), a 

 middle mesenchymal layer (angioblast), and a 

 covering of mesothelium. X 240. 



city of germ layers. Such 

 position is hardly tenable in 

 view of the fact that origin- 

 ally there is only one germ 

 layer, the ectoderm, and the 

 origin of the mesoderm from 

 the primitive streak, a phase of fusion of ectoderm and entoderm. More- 

 over, in certain forms, e.g., Tarsius, a primate, it is claimed by Hubrecht 

 that mesoderm has a double origin from ectoderm and entoderm, as well as 

 from primitive streak. Again in subsequent early embryonic stages it is 

 believed by many that blood-cells arise by process of differentiation of body 

 mesenchyme. In the further discussion of hemopoiesis we may take for 

 granted the origin of blood exclusively from mesenchymal elements. 



Regarding the question of the original blood mother-cell two sharply 

 contrasting views are held, expressed in the monophyletic (unitarian) and 

 polyphyletic (dualistic) theories of blood-cell origin. The former holds 

 that all types of blood elements, red and white, have their origin directly 

 in one and the same primordial blood cell ('hemagonium/ hemoblast) ; the 



