90 THE EARLY DEVELOPMENT OF MAMMALS. 



cellular space. By the migration of the cells and their multiplication, the mesen- 

 chyma is produced. It fills up all the room between the mesothelium and the two 

 primary germ-layers so far as it is not occupied by the developing blood-vessels and, 

 later, by lymph vessels. 



Apparently the entire mesothelium may participate in the production of the 

 mesenchymal cells. Its different regions, however, do not so participate all to an 

 equal degree, or at the same time. The throwing off of mesenchymal cells may be 

 observed in certain parts of the embryo in somewhat advanced stages of develop- 

 ment, and it seems not impossible that the process may be found to occur even 

 in adult life. 



The mesoderm, by the formation of mesenchyma, becomes very early unlike 

 the other germ-layers. Both ectoderm and entoderm are epithelial membranes. 

 The mesoderm is partly epithelial, partly mesenchymal, and from the mesenchyma 

 arise special kinds of tissue which are characteristic of the middle germ-layer, and 

 never are produced from either the outer or inner germ-layers. 



The Origin of the Blood-vessels and Blood. 



As stated above (pages 66 and 80), the angioblast and first blood-vessels ap- 

 pear in the circumscribed region in the mesoderm of the yolk-sac and lie close 

 against the entodermal cells of the area opaca. The region which they occupy is 

 termed the area vasculosa. From the area vasculosa the development of blood- 

 vessels extends, as stated, across the area pellucida into the embryo.* During these 

 early stages the only blood-vessels are in the splanchnopleure. After their formation 

 has extended into the body of the embryo, it spreads into the somatopleure also, 

 which, therefore, acquires its blood-vessels at a later stage. It should be noted, 

 however, that the development of the blood-vessels begins before the ccelom has 

 been developed over the area vasculosa. While they are forming, the ccelom 

 expands; and after it has appeared, the primitive blood-vessels are found always 

 exclusively in the splanchnic mesoderm. 



Definition. The essential part of a blood-vessel is its endothelial wall. In 

 early stages all the blood-vessels consist only of endothelium. Arteries and veins 

 differ but little, if at all, in histological structure during early embryonic stages, and 

 are distinguished chiefly by the direction of blood-currents passing through them. 

 Capillary blood-vessels and sinusoids have, as a rule, throughout life merely an en- 

 dothelial wall. Arteries and veins become strengthened by the development of 

 special coats around the endothelium which arise by transformations of the mesen- 

 chymal cells in the immediate neighborhood of the vessels. 



The Development in the Chick. The first indication of the blood-vessels is a ret- 

 iculate appearance, which can be recognized in the mesoderm in surface views of 

 the fresh or hardened embryo at the end of the -first day. The reticulate structure 

 increases rapidly in extent and distinctness during the second day of incubation. 



* It has been recorded that in lizards the vascular anlages appear first in the area pellucida. 



