ERYTHROBLASTS IN THE PIG EMBRYO 187 



c. The erythrocytic nuclei remaining after the completion of 

 such a cytoplasmic constriction may be surrounded by an appre- 

 ciable quantity of cytoplasm, in other cases there may be little 

 if any cytoplasm evident. 



d. In size and hemoglobin content, these newly formed cor- 

 puscles are comparable to the normal plastids. As observed in 

 the cultures, the majority when first formed were more or less 

 spherical in shape, others manifest a tendency to become flattened, 

 and some evidence was obtained in which a cup-shape was assumed 

 while under observation in the cultures. An occasional plastid 

 possessed a tapering cytoplasmic process which was observed to 

 rise in connection with the final separation of the corpuscles from 

 the parent cell. 



e. A critical analysis of the technique, plasma, thermal and 

 physical conditions in the cultures, together with the stage of the 

 embryo, the normal and degenerative cytological changes and 

 staining reactions of the erythrocytes to the end of ascertaining to 

 what extent the above described behavior of the erythroblasts is 

 to be regarded as normal or abnormal, indicates that the culture 

 conditions are such as to justify anticipating some manifestation 

 of the normal transitional stages in the origin of plastids. 



3. Evidence concerning the origin of the plastids in the embryo 

 by a similar process : 



The occurrence in carefully fixed blood vessels of erythroblasts 

 having a constriction of the cytoplasm between the nuclear and 

 cytoplasmic poles, the variation in the size and form of the plastids, 

 the presence of pyknotic erythrocytic nuclei surrounded by a rim 

 of hemoglobin-containing cytoplasm, together with the observa- 

 tion in fresh vessels of the separation off of a cytoplasmic part 

 of the erythroblast by a process of cell constriction, is evidence 

 strongly indicating the origin normally of plastids in the embryo 

 by a process of cytoplasmic constriction similar to that observed 

 in vitro. 



4. General considerations regarding the origin of non-nucleated 

 erythrocytes : 



a. Concerning the prevalent views of nuclear extrusion and 

 intracellular disintegration, arguments so far as they are based 



