ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 285 



larly formed (figs. 25 and 35). The bi- and multinucleated 

 types will be further described under 'giant cells.' 



Haemoblasts have a double source of origin: 1) from the 

 mesenchyma (fig. 30) ; 2) from the endothelium of the earliest 

 blood vessels (fig. 4). Since this endothelium, however, also 

 originally arose from mesenchyme, the primary, in part indirect 

 source, is the same, namely the original mesenchyma. 



The endothelial origin of haemoblasts has already been 

 partially described above under 'endothelium.' It need merely 

 be emphasized here that the evidence on this point seems un- 

 equivocal; transition stages are practically innumerable; their 

 abundance is so great as to make it difficult to adhere to a 

 reasonable limit in the selection of illustrations. Possible 

 objections to the interpretation here given to the observations 

 will be considered below. The above description pertains only 

 to intravascular haemopoiesis; the endothelium contributes 

 also, but apparently much more rarely (except in the mesoneph- 

 ric glomeruli of the body of the embryo) , extra vascular haemo- 

 blasts. The continuity of such with the endothelial wall counter- 

 vails the possible objection that these are migrants (fig. 4). 



The direct mesenchymal origin of haemoblasts concerns it- 

 self with the blood-islands and certain isolated cells separating 

 from the mesenchymal syncytium. Peripherally the blood- 

 islands are in continuity with the mesenchyma, where endothelial 

 cells are differentiated; centrally the cells are haemoblasts in 

 various earlier stages of metamorphosis into erythroblasts ; 

 some of these may be binucleated (fig. 29). 



The unique and crucial evidence for mesenchymal origin of 

 haemoblasts pertains to certain isolated cells caught in the 

 actual process of differentiation and separation from the syncy- 

 tium. These are admittedly rare, but the evidence they furnish 

 is of prime importance. It supplies the link in the monophyletic 

 theory of haemogenesis concerning which there has been the 

 greatest scepticism. Figure 30 is an illustration of the clearest 

 case of the condition referred to. Here is shown an area of 

 mesenchyma in which two of the nuclei, as well as their envelop- 

 ing cytoplasm, have mesenchymal features; the third nucleus 



