CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 91 



(a) in figures 18 and 19 present nuclear and cytoplasmic char- 

 acteristics readily comparable to the erythrocytes to be found in 

 the adjacent blood vessels, although this is by no means so clear- 

 ly evident in such cells as (c), figure 18. In view of the recent 

 observations of Haff ('14) and Bremer ('14) indicative of a par- 

 ticipation of mesothelial cells in the origin of blood islands and 

 blood cells it would indeed not appear so remarkable if the same 

 cells liberated into the coelomic cavity would here be found to 

 manifest a potentiality for erythrocytic differentiation. How- 

 ever, it must be admitted that no convincing evidence of such a 

 differentiation was obtained. The irregular form and structural 

 character of the nuclei of these cells is not typical of develop- 

 ing erythroblasts, the nuclei of which during the earher phases 

 of their differentiation are normally more or less spherical in 

 shape (Emmel '14). No evidence of phagocytic activity as a 

 characteristic which might justify associating these cells with 

 the coelomic macrophags was observed, nor is the vacuolated 

 and fragmented condition of the cytoplasm in many of these 

 cells suggestive of progressive erythrocytic differentiation. 



On the contrary the following considerations support a dif- 

 ferent conclusion. As already indicated wherever these eosin 

 staining cells are found in groups, some of the cells in such a 

 group can almost always be clearly identified as erythrocytic. 

 It is equally evident that in the case of erythrocytic degenera- 

 tion, the nuclei (i.e. of erythroblasts) may undergo form changes 

 identical with those to be observed in these eosin staining cells. 

 Such a lobulation and subdivision of erythrocytic nuclei can not 

 infrequently be found even in the circulating blood (fig. 37 and 

 the subsequent description on p. 100). Weidenreich ('03, p. 

 420) has fully described degenerative changes in which the eryth- 

 rocytic nucleus becomes irregular in form, indented, bilobed, 

 dumb-bell and clover leafed shaped, and finally constricted into 

 two or more parts connected by a small thread-like strand or 

 entirely separated from each other and thus give rise to a so- 

 called double nucleated cell. In some cases such degenerating 

 nuclei may become smaller, more compact and take a much 

 darker stain, in other instances, however, the nucleus may main- 



