128 VICTOR E. EMMEL 



3. Critical discussion of the cultures and cytoplasmic constrictions 160 



a. Plasma, temperature and other conditions 161 



b. The erythrocytes: normal and degenerative changes 163 



4. Evidence concerning the origin of plastids in the embryo by a similar 



process of cytoplasmic consti'iction .- 166 



a. In the living vessels 166 



b. Infixed vessels 168 



c. Size and form of the plastids 169 



d. Erythrocytic nuclei after plastid formation 171 



Discussion concerning the origin of non-nucleated erythrocytes 173 



1. The question of nuclear extrusion and intra-cellular disintegration. . . . 173 



2. Evidence from previous investigators bearing on the origin of plastids 



by cytoplasmic constriction 177 



a. In mammals 177 



b* In other vertebrates 180 



3. The role of cytoplasmic constriction in the origin of other non-nucleat- 



ed morphological elements of the mammalian blood 182 



Summary and conclusions 185 



Literature cited 189 



INTRODUCTION 



The results presented in the present paper are concerned, first, 

 with certain morphological changes in the structure of the eryth- 

 roblast previous to the origin of the non-nucleated erythrocytes 

 or plastids, 1 and second, with the mode of origin of the hemo-globin- 

 containing plastids from these highly differentiated nucleated 

 cells. Attention will be directed primarily to such observa- 

 tions which, so far as it has been ascertained from the literature 

 of the subject, appear to contribute, in a degree at least, additional 

 facts regarding the blood of the pig embryo, together with a dis- 

 cussion of the bearing of this data upon certain problems con- 

 cerning the cytomorphosis of the mammalian red blood corpuscle. 



The material consisted of 3 to 40 mm. pig embryos. The 

 methods employed in the study of fresh and fixed blood vessels, 

 and blood cultures will be indicated in the course of the ensuing 

 description. 



' The term 'erythrocyte' is here used in the sense adopted by Professor Minot 

 ('13), in the Keibel-Mall Embryology "as a collective term for any and all red 

 cells," while the term 'plastid' is employed to designate all non-nucleated red blood 

 corpuscles. 



