86 CHARLES JR. STOCK ARD 



The question involved is then: Which is correct, the mono- 

 phyletic or polyphyletic theory of haematopoesis? It is recog- 

 nized by all that both propositions are classed only as theory. 

 It must further be recognized that both theories are based at 

 the present time only upon the interpretations of various ob- 

 servers, these interpretations are not necessarily facts. I trust, 

 therefore, that the experiments on Fundulus embryos may add 

 a basis of unquestionable facts which may show the correctness 

 of one or the other of these interpretations. 



With this point of view, we may undertake a critical examina- 

 tion of the evidence so ably presented by Maximow ('09) in his 

 study on the mammalian embryo. The observations he con- 

 strues as strong argument in favor of the monophyletic origin of 

 all types of blood cells and vascular endothelium. This contri- 

 bution by Maximow ('09) has been accepted by many embryolo- 

 gists d bras ouverts, and has been largely incorporated into several 

 recent chapters on the development of the blood, for example, 

 by Schaefer ('12), and Minot ('12). 



In the primitive streak stage of the rabbit embryo Maximow 

 states that the peripheral mesoderm in which the blood islands 

 will later occur has in no sense the character of a connected 

 epithelial layer, but consists merely of local accumulations of 

 cells of mesenchymatous type. The cells of this mass are of long 

 thin spindle shape or with star-like processes. These cells are 

 probably much of the same type as the wandering cells seen on 

 the yolk-sac of the Fundulus embryos. In this peripheral mesen- 

 chymatous mesoblast the first blood islands arise in the caudal 

 part of the area opaca, as originally described by Van der Stricht. 

 The blood islands are formed from the spindle or branched mesen- 

 chyme cells which become associated into groups. 



Maximow states that the first endothelial cells like the pri- 

 mary blood cells are also derived from the mesoblast-mesenchyme. 

 With this one may fully agree and several other tissues could 

 be included in the statement as derived from mesenchyme. 

 Maximow, however, goes further and thinks this general source 

 a common specific source. Thus the endothelial cells and blood 

 cells are closely related and arise from a common stem cell in 



