THE DEVELOPMENT OF THE VASCULAR SYSTEM. 275 



finely granular cytoplasm — mononuclear leucocytes; the nuclei of the mononu- 

 clear forms become drawn out into horse-shoe shapes characteristic of the transi- 

 tional variety ; the nuclei of the transitional forms then become lobed or separated 

 into two or more parts to form the nuclei of the polymorphonuclear or poly- 

 nuclear varieties. In this case, therefore, the different varieties of leucocytes 

 bear definite genetic relations to one another, the polymorphonuclear and 

 polynuclear forms indicating senility and the beginning of degeneration. 



The more recent studies on the formation of blood cells (Maximow and 

 others) have to a considerable extent discredited the view that the different 

 varities of white blood cells, from lymphocytes to polynuclear forms, bear 

 definite genetic relations to one another. While they are all derived from in- 

 different mesenchyme cells, yet they do not fall into a series each member of 

 which bears a direct genetic relation to the preceding and succeeding member, 

 as Gulland maintained (see preceding paragraph). Maximow has demon- 

 strated the origin of lymphocytes not only in the area vasculosa but also on 

 the body mesenchyme. He holds, furthermore, that the granular forms of 

 leucocytes arise only in the body mesenchyme. 



In the bone marrow the indifferent mesenchyme cells may give rise to 

 several different kinds of cells. They may differentiate into osteoblasts, which 

 by fusion produce osteoclasts (polykaryocytes). As already stated, they may 

 become modified to form lymphocytes which in turn may give rise to red blood 

 cells. They may produce the various members of the myelocyte series and 

 leucocytes of all varieties. The genetic relations of myelocytes and leucocytes 

 have not been clearly made out. From the mesenchyme cell may be derived 

 also the type of cell known as the megakaryocyte, from which, according to 

 Wright, the blood plates are broken off (see below). (Fig. 250.) 



What part is played by the lymphoid organs in the production of the different 

 kinds of white blood cells is a matter of some doubt. It is certain, however, 

 that lymphocytes multiply in these organs, especially in the germinal centers in 

 the lymph glands, and it is very probable that they differentiate in situ out of 

 mesenchyme cells in the developing lymph glands (see p. 280). At one time 

 Beard attempted to discredit the mesenchymal origin of leucocytes by his 

 studies on the thymus in lower vertebrates. He asserted that he found no 

 leucocytes in the blood before the appearance of the thymus, and that the 

 primitive leucocytes were apparently derived from the epithelial (entodermal) 

 cells which constitute the anlage of the thymus. The recent researches of 

 Maximow have disproved Beard's theory. 



The origin of the blood plates is even more obscure than the origin of the 

 blood cells. 1. The theory that they represent products of disintegration of 

 leucocytes has not been corroborated. 2. The view that the plates stand in 

 genetic relation to the erythrocytes is supported by the fact that the latter can 



