CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 111 



b. The ring-like configuration presented by many of these 

 degenerating erythrocj^tic nuclei appear identical in many re- 

 spects with the nuclear structures occurring in the erythrocytes 

 of pathological blood known as Cabot's rings and they conse- 

 quently furnish evidence that the latter are not limited to the 

 blood of the adult animal as has been previously assumed 

 (Naegeh). 



c. Certain cellular elements in the mesenchyma present in 

 some respects the appearance of eosinophilic leucocytes and are 

 suggestive of a possible mesenchymal origin as maintained by 

 Maximow. In view of the fact, however, that all of these are 

 deficient in any definite leucocytic granules and that nucleated 

 red blood corpuscles escape into the embryonic tissue spaces 

 where they may present degenerative nuclear and cytoplasmic 

 characteristics apparently identical with those of the non-gran- 

 ular eosin staining cells in the mesenchyma, it appears difficult 

 to escape the conclusion that the majority of the latter, just as 

 in the case of the corresponding coelomic elements, are degen- 

 erating erythrocytes rather than granular leucocytes develop- 

 ing in situ from mesenchymal cells. 



6. Concerning the problem of mesothelial origin of macro- 

 phags. 



a. In connection with present conclusion that the coelomic 

 epithelium may give rise to free functional elements in the coe- 

 lomic cavities, it appears not without significance on embryo- 

 logical and comparative grounds to note the recent work of Brem- 

 er, Haff and Schulte concerning the participation of coelomic 

 epithelium in vasculogenesis and the formation of blood cor- 

 puscles in vertebrate embryos and the conditions in certain in- 

 vertebrates where the coelomic epithelium gives rise to cellular 

 elements functioning as respiratory and phagocytic cells in the 

 body cavities (Lang, His, Arnold). 



b. Regarding the origin of the macrophags in the serous cavi- 

 ties of adult mammals unanimity still remains to be attained in 

 the solution of the problem. On the basis of the data so far at 

 hand, it does not appear, however, that a convincing case has 

 as yet been made against the possible persistence of a potential- 



