SPECIAL HISTOLOGY. 



I. BLOOD AND BLOOD-FORMING ORGANS, HEART, 

 BLOOD-VESSELS, AND LYMPH-VESSELS. 



A. BLOOD AND LYMPH. 



J. FORMATION OF BLOOD. 



EARLY in the development of the embryo there appear in a por- 

 tion of the extra-embryonic area of the blastoderm, known as the 

 area vasculosa, definite masses of cells, derived from the mesen- 

 chyme, and spoken of as blood islands, which are intimately connected 

 with the formation of the blood. If these blood islands be examined 

 at a certain stage, free cells are seen lying in their center, appar- 

 ently derived from the central cells of the islands ; the cells sur- 

 rounding them represent the elements which later go to form the 

 primitive vascular walls. The free elements are the first blood-cells 

 of the embryo. The blood-cells thus developed enter the circula- 

 tion by means of blood channels formed by the confluence of the 

 blood islands. These grow toward the embryo and later join the 

 large central vessels. The origin of these blood islands is still an 

 open question. Some authors contend that they arise from the 

 mesoblast (P. Mayer, 87, 93 ; K. Ziegler ; van der Stricht, 92), 

 others that they are of entodermic origin (Kupffer, 78 ; Gensch ; 

 Riickert, 88 ; C. K. Hoffmann, 93, I ; 93, II ; Mehnert, 96). At a 

 certain period the embryonic blood consists principally of nucleated 

 red cells, which proliferate in the circulation by indirect division. 

 The colorless blood-cells, the development of which is not yet fully 

 understood, appear later. It is possible that they also are elements 

 of the blood islands, which do not contain any hemoglobin. In a 

 later period of embryonic life the liver becomes a blood-forming 

 organ. Recent investigations have, however, shown that it does 

 not take a direct part in the formation of the blood, but only 

 serves as an area in which the blood-corpuscles proliferate during 

 their slow passage through its vessels. The blind sac-like endings 

 of the venous capillaries seem to be particularly adapted for this 

 purpose, as in them the blood current stagnates, and it is here that 

 the greater number of blood-cells reveal mitotic figures. The 

 newly formed elements are finally swept away by the blood stream 

 and enter the general circulation (van der Stricht, 92 ; v. Kostan- 

 ecki, 92, III). Many investigators believe that the red blood-cells 



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