DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 155 



type of blood cells other than erythrocytes in such a position. 

 The same is true of the cellular products of the intermediate 

 cell mass; in hundreds of sections studied with the oil immersion 

 not one case has been found of a lymphocyte or leucocyte arising 

 from a cell of the original intermediate cell mass. It is thus 

 concluded that this mass is the red-blood anlage of the teleost 

 and the wandering cells of the yolk-sac which are very probably 

 derived from the same mesodermal stem that forms, the inter- 

 mediate cell mass are likewise a portion of the red blood cell 

 anlage. 



The embryos that fail to develop a circulation are most im- 

 portant material for the study of these questions of relationship 

 among the different types of blood cells. The observations on 

 the living show definitely the qualities of the early blood forming 

 mesenchymal cell in its assumption of the globular slowly wander- 

 ing type, which would fully satisfy the descriptions of Maximow's 

 'primitive blood cell' as being closer in appearance to a lymph- 

 ocyte than to a red corpuscle. When one follows these supposedly 

 indifferent primitive blood cells which are claimed to possess 

 the power of differentiating into either a leucocj^te or an erythro- 

 blast, he invariably observes them to differentiate only into 

 erythrocytes. Although all cells of the early islands are dis- 

 tinctly visible, they are not mixed in type, but are of one class. 

 A long study of these early islands in sections also fails to reveal 

 any other than red blood cells. The leucocytes are not very 

 numerous in the blood, yet they should be seen if present in these 

 islands, as they are found in other positions and are well 

 represented in the blood of the adult Fundulus. 



The further history of the cells in the stagnant masses on the 

 yolk-sac of an embryo in which the blood has never circulated 

 is instructive in several ways. In the first place, all of these 

 islands seem to become surrounded by endothelium, yet the 

 endothelial arrangement cannot be completely traced in every 

 case and it rarely extends much bej^ond the island mass so far 

 as one can observe in life. 



Pigment cells are irregularly scattered in the neighborhood 

 of the islands, as they are throughout the yolk region. The 



