140 VICTOR E. EMMEL 



examined in vitro, that the nucleus "reste longtemps accole a V 

 espece de cuticule qu' il vient de quitter" (p. 264). In mammals 

 the occurrence in the circulation of erythrocytes with these cyto- 

 plasmic processes appears to be limited to the embryo. At least 

 the writer is not at present aware of similar observations for 

 the adult blood, except under certain pathological conditions as 

 in cases of anemia. A possible explanation suggests itself for this 

 fact is that in the embryo, in contrast to the adult, mitosis and 

 cytoplasmic constriction in the erythroblasts takes place directly 

 within the embryonic blood vessels. Here merely the passive 

 movements of the blood corpuscles as they are whirled along in 

 the blood current may be expected to interfere with the normal 

 completion of the final stages of cytoplasmic separation or con- 

 striction in the dividing erythroblasts, especially in the later stages 

 of their cytomorphosis, whereas in the adult these processes take 

 place in a comparatively quiet environment, such as that of the 

 red bone marrow. 



3. ORIENTATION OF MATURE ERYTHROBLASTS WITH REFERENCE 



TO GRAVITY 



Attention has already been called to the fact that the biconcave 

 disc-shaped erythroblasts of the 25 to 35 mm. pig embryos tend 

 to assume a flat position upon the horizontal surface on which 

 they are resting, whether this surface be that of a glass slide or 

 the wall of a blood vessel. A further interesting observation re- 

 garding the orientation of these erythroblasts which, so far as 

 I have been able to ascertain, has not yet been elsewhere described, 

 remains to be presented. 



In mechanically turning the cells over by means of a gentle 

 and repeated tapping on the cover-glass it was discovered that, 

 in the case of the nucleated discs with the nucleus eccentric in 

 position, the edge of the disc which rises first from the horizontal 

 surface of the glass slide is the part opposite the nucleus (fig. 

 21, a and b). If the disturbance is not too vigorous and the cell 

 is observed closely as it turns, this raised part of the rim of the disc 

 will gradually approach the observer until only a profile view 



