294 H. E. JORDAN 



meseiichynia of tlie body stalk in the form of cords and tubules 

 (angiocysts) the cells of which differentiate into haemoblasts and 

 endothelium. Bremer's observations, however, are a further 

 \'ery strong support to the claim that angioblast is of mesenchy- 

 mal origin, and that mesenchyma, mesothelium and endothelium 

 are originally identical structures. 



The monophyletic theory of blood cell origin considers the 

 haemoblast the common parent of both erythrocytes and leuco- 

 cytes. Its correspondence with fact, at least in essential out- 

 lines, is now widely accepted. The point which has stimulated 

 most discussion concerns the origin of isolated haemoblasts 

 within the mesenchyma. Are such differentiation products of 

 the mesenchyma, or are they migrants from the blood vessels? 

 The latter view was held by Minot (19); Maximow (16 and 17) 

 and others champion the opposing view. In the case of the yolk- 

 sac of the pig, the evidence seems definite in favor of the in situ 

 differentiation of haemoblasts from the mesenchyma. The 

 observations both from blood-islands and single cells have been 

 given above. Haemoblasts of course are capable of amoeboid 

 activity, and undoubtedly do leave the blood vessels under 

 certain conditions, and invade the surrounding mesenchyma. 

 But that the cell (h) illustrated in figure 30 cannot be interpreted 

 as such is clear from: 1) the connection of its nucleus, through a 

 delicate chromatic bridge, with the nucleus of the mesenchyma; 

 and 2) its perfectly healthy condition, both from the viewpoint 

 of its nucleus and its cytoplasm. Nor can there remain any 

 doubt that it is actually a haemoblast when its cytoplasm and 

 luicleus, in contrast to the cytoplasm and nucleus of the mesen- 

 chyma, is compared with an intravascular haemoblast. 



The evidence given above for the extensive origin of haemo- 

 blasts from the endothelium seems conclusive for the 10 mm. pig 

 embryo. Neither at earlier nor later stages is this jirocess so 

 evident. 



The haemogenic activity of the endothelium in the yolk-sac 

 of the pig is of cardinal significance especially in view of Stock- 

 ard's (23) findings in the case of the Fundulus embryo, where the 

 problem was approached by the experimental method. This 



