ERYTHROBLASTS IN THE PIG EMBRYO 147 



of such a change in the weight of the nucleus, emphasizes an in- 

 crease in hydrostatic pressure as a factor in both the assumption 

 of an eccentric position as well as in the ensuing extrusion of the 

 nucleus. The question still seems an open one concerning which 

 some of the observations in the present study appear to justify 

 a further discussion. 



Concerning the explanation of this eccentricity on the basis 

 of the nucleus lying in a fluid or semifluid medium, no evidence 

 has so far been attained for the pig embryo, that the nucleus of 

 the mature erythroblast is thus free inside the cell, nor was it 

 ever observed to change its position during any mechanical 

 manipulation of the blood corpuscles. On the other hand, some 

 observations were made indicating a more or less fluid condition 

 of the hemoglobin containing cytoplasm as illustrated in figure 35, 

 where, in certain cells being studied in vitro, occasionally what 

 appeared to be a vacuole was observed to move about in the cy- 

 toplasm. In the case shown in the figure there could be no ques- 

 tion that the vacuole was located within the cell, as was deter- 

 mined by a mechanical rotation of the cell, nor could there be any 

 doubt but that the vacuole actually changed its relative position 

 for the corpuscle itself appeared to remain stationary. Concern- 

 ing the assumed decrease in the specific gravity of the nucleus, 

 the preceding observations regarding orientation in which the 

 nuclear pole tended to swing undermost, while not conclusive, 

 would if anything suggest a greater rather than a lesser weight 

 for the nucleus as compared with an equal volume of hemoglobin 

 containing cytoplasm. 



On the other hand, some of the present observations are of a 

 more positive character with reference to this question and have 

 an interesting bearing on the problem. It will be recalled that 

 in the previous description of the behavior of the cells in changed 

 osmotic conditions that the reactions were of such a character 

 as to indicate structural differences between the nuclear and 

 cytoplasmic poles of the mature erythroblast. Such a structural 

 difference appears further indicated by the occurrence in the 

 circulation of the pig embryo (cf. p. 171) of so-called free erythro- 

 cytic nuclei surrounded by a rim of cytoplasm (figs. 10, 11 and 12), 



