188 VICTOR E. EMMEL 



upon the indirect evidence from free erythrocytic nuclei in the 

 circulation, erythrocytic nuclear inclusions in phagocytic cells, 

 and nuclear particles in plastids, do not appear conclusive for 

 either theory and seem equally well accounted for on the basis 

 of cytoplasmic constriction. Nor is the more direct evidence for 

 nuclear extrusion, at least in the embryo, sufficiently convincing 

 but that the problem appears still to be an open one concerning 

 which the present data for cytoplasmic constriction may be justi- 

 fiably introduced. 



b. Evidence from other mammals directly or indirectly sup- 

 porting the present observations and conclusions is furnished, 

 among others, by Malassez, Janosik, Engel and Howell. Similar 

 evidence for the lower vertebrates by Giglio-Tos, Eisen, Engel 

 and Jolly, appears of more than passing significance if the non- 

 nucleated erythrocytes normally occurring in lower vertebrates are 

 to be regarded as phylogenetically precursors of the respiratory 

 elements which predominate in the adult animal. 



c. The origin of non-nucleated erythrocytes by cytoplasmic 

 constriction, if correct, renders the erythroblasts in this respect 

 comparable with the megakaryocytes, plasma cells, and lympho- 

 cytes, to which they are apparently closely related genetically, 

 for according to recent investigations these cells normally con- 

 tribute to the mammalian circulation at least two important 

 non-nucleated morphological elements other than plastids, by the 

 constriction off of parts of their cytoplasm. 



5. In conclusion, therefore, it may be stated that the data de- 

 rived from the present investigation involving the study of blood 

 cultures, living and fixed blood vessels in the pig embryo, together 

 with the observations of other investigations for both red and 

 white blood cells in various mammals, raises the question whether 

 the origin of non-nucleated erythrocytes by a process of cytoplas- 

 mic constriction does not merit more serious consideration. 



It is a pleasure to express here my appreciation of the encourage- 

 ment received from Prof. R. J. Terry during the investigation 

 and for valued criticism in the completion of this paper. 



