158 VICTOR E. EMMEL 



these processes may also be observed to constrict off and become 

 detached from the cell body (fig. 17, e to s), thus leading to a 

 further reduction of the cytoplasm which in some cases may even 

 result in leaving a practically cytoplasmic-free nucleus. 



Fourth, as to the size of the plastids. The diameter of the plastids 

 arising from such constrictions as illustrated in figures 14, 15 and 

 18 is practically the same as the average non-nucleated erythro- 

 cytes of the embryonic pig blood. Other plastids, although similar 

 in origin, may vary greatly in size on either side of this average. 

 Some are much larger (fig. 16), and equivalent in size to the 

 so-called macrocytes of the circulation, others again are much 

 smaller than the average non-nucleated erythrocyte, and cor- 

 respond in size to the microcytes. Some of the smaller of these 

 plastids which may be about one third or one fourth the size of 

 the average non-nucleated erythrocyte, or about the dimensions 

 of blood platelets, may arise as small cytoplasmic buds or constric- 

 tions from a typical erythroblast, or they may be formed as a 

 further subdivision of a larger plastid (fig. 17, l-s). In the latter 

 case, the cytoplasm of the large plastid, when first liberated from 

 the parent erythroblast, not infrequently exhibits a quivering 

 modification of its contour, especially at the point where the 

 separation has occurred. A secondary constriction may arise at 

 this region of separation and cut off one of the small elements un- 

 der consideration, the activity in the remainder of the large plastid 

 then ceases and the corpuscle assumes a stable, quiescent condi- 

 tion. These smaller bodies frequently manifest a considerable de- 

 gree of activity, as indicated by slight variations in shape, before 

 coming to rest in a more or less stable rounded form. 



Fifth, as to the form of the plastids. Since the present experi- 

 ments consisted chiefly of hanging drop cultures, concerning the 

 determination of the shape of the plastids, an element of uncer- 

 tainty is introduced on account of the tendency of flattened cor- 

 puscles to assume a horizontal position, in which case they may 

 present a deceptively rounded appearance (cf. p. 132). As a rule 

 the plastids whose origin had been directly observed in the cul- 

 tures, when first formed appeared to have a more or less spherical 

 shape. In order to make more positive observations a further 

 study was made of experiments in which the culture was enclosed 



