ERYTHROBLASTS IN THE PIG EMBRYO 139 



take place in the blood even while actively circulating in the 

 vessels. Figure 9 represents two instances of mitosis in circu- 

 lating blood in which the two daughter cells are still connected by 

 a strand of cytoplasm (cf. also fig. 42). It is readily seen that 

 a final separation of this strand without an immediate rounding 

 up of the newly formed cells would give rise to tapering, point- 

 ed cytoplasmic structures similar to those under consideration. 

 Processes in the case of the non-nucleated plastids may also arise in 

 a somewhat similar manner by cytoplasmic constrictions, for, 

 as will be described later in the study of blood cell cultures, ery- 

 throblasts were observed to undergo a process of constriction into 

 two parts, the one nucleated, the other non-nucleated and similar 

 in appearance to a plastid; considerable evidence is at the same 

 time also advanced for the normal occurrence of a similar phe- 

 nomenon in the circulation. Occasionally plastids arising in this 

 way were observed to constrict off in such a way as to leave 

 a slender cytoplasmic process projecting from the surface in the 

 region where the final separation from the parent erythroblast took 

 place (figs. 20, 17, 13 and 32). The foregoing observations seem 

 to offer an adequate explanation for all of these cytoplasmic struc- 

 tures with the exception of a few instances where a cell may have 

 more than a single process or a single process is partly bifurcated, 

 in which case possibly other factors may be involved. 



Concerning the persistence of cytoplasmic strands during the 

 division of the cytoplasm of the cell body. Jolly ('04) records the 

 observation of a similar phenomenon in his study in vitro of the 

 red blood cells of triton. In describing the division of these cells 

 he states '41 arrive enfin un moment 6u les deux cellules ne sont 

 plus reliees que par un pont tres mince et tres court qui ne tarde 

 pas a se rompre. II peut se faire que ce pont persiste un certain 

 temps, mais c'est absolument exceptionnel" (p.* 621). In older 

 cultures he found that sometimes these strands may be some- 

 what longer and thicker and persist for a longer time, in which case 

 he infers that the cytoplasmic activity necessary for the complete 

 separation has been interfered with. It is of interest, also, to note 

 that in figure 17 of Jolly's ('07) monograph, which is given to 

 illustrate nuclear extrusion in the bone-marrow from the white rat 



