DEVELOPMENT OF HUMAN PHARYNX 369 



pretation. It is possible, however, to look at the unity in origin 

 of the blood cells as an expression of a fundamental unity of con- 

 ditions. While this view would not necessarily oppose the mono- 

 phyletic interpretation under either an angioblastic or mesen- 

 chymal origin, it would not, on the other hand, necessarily 

 presuppose it. One of the characteristic features of hemato- 

 poiesis is the ever-shifting setting of the scene, the succession 

 of regions in which blood-cell formation occurs: — blood islands 

 (yolk sac), liver, par- and preaxial mesenchyme (?), mesonephros 

 (some forms), spleen, lymphatic tissue, thymus, bone marrow^ — 

 as well doubtless as other places. There may well be a common 

 metabolic feature that determines the appearance of blood- 

 cell formation successively or simultaneously in these different 

 places. That 'absorption'- — frequently — of products of regres- 

 sive change may be an element is suggested, as illustrated, 

 for example, by the tendency of blood-cell formation to follow 

 bone (or cartilage) absorption even when it occurs in very 

 unusual localities (Maximow '07 b). On the other hand, it is 

 possible that the blood-forming regions are characterized by 

 a continued growth tendency in the mesenchyme, associated 

 with a lack or loss of the adaptive connective tissue differentiation, 

 such as occurs in other places such as, for example, the formation 

 of skeletal connective tissue membranes in correlation with 

 epithelia, etc., and that this covers factors that determine hema- 

 topoiesis. Our knowledge as yet is clearly neither detailed 

 enough nor extensive enough to permit any comprehensive 

 conclusions. In the case of the thymus, however, not only 

 lymphocyte formation but also the formation of granular leu- 

 cocytes (including eosinophile cells) and red blood corpuscles 

 incontestably occur, although I question whether many of the 

 last named find their way into the general circulation (cf . Badert- 

 scher '15, b). It should also be noted that formation of granu- 

 lar cells and red blood corpuscles occurs in the cervical vesicle 

 (ectodermic) portion of the thymus in the pig (thymus super- 

 ficialis), as well as in the entodermic thymus. 



To return to the morphological interpretation of the thymus : 

 It is obvious that an explanation from the point of view set forth 



