ERYTHROBLASTS IN THE PIG EMBRYO 135 



that during cytomorphosis the erythrocytic nuclei undergo a 

 reduction in size. This decrease in volume together with the 

 associated increase in the density of the stain and compactness of 

 the chromatin may be noted in figures 1, 2, 3, and 4. It will fur- 

 thermore be observed in these figures that in the more fully 

 differentiated cells the nuclei have come to occupy an eccentric 

 position in the corpuscle. 



In addition to these well known nuclear changes there is an 

 observation to be described concerning the form of the nucleus in 

 the nature of mammalian erythroblasts which does not appear to 

 have been previously noted, at least for the pig embryo. In the 

 3 mm. pig nuclei of the primitive erythroblasts are large round 

 structures, containing scattered granules and masses of chroma- 

 tin (fig. 1). It will also be observed that these large nuclei occupy 

 the major part of the cell body. In embryos up to 10 mm. or even 

 older, there is little evident deviation from the spherical form 

 of the nuclei in the developing erythroblasts. On the other hand, 

 in still older embryos — or what is more to the point, in the more 

 highly differentiated corpuscles of young embryos — it appears 

 that a departure from this form may occur. Although the nuclei 

 as viewed in the mature erythroblast with the disc-like corpuscle 

 lying in a flat, horizontal position, as in the ordinary cover-glass 

 preparations, may still appear round, if, however, the corpuscle is 

 rotated so as to present a profile view of the disc, a decided flat- 

 tening of the nucleus may frequently be observed (figs. 3, 4, 6, 22 

 and 25). In other words, in conformity with the discing of the 

 erythroblasts there is a tendency in at least the majority of the 

 mature cells toward a correlated flattening of the nuclei. This 

 flattening in nuclear form is evidently not to be confused with 

 nuclear constrictions or fragmentation. The short axis of such a 

 nucleus tends to lie in the same plane as that of the short axis of 

 the disced corpuscle. In some instances where the cell body has 

 already approached the definitive form, while the nucleus is still 

 more or less rounded, there may remain a slight cytoplasmic 

 bulging or thickening of the rim of the disc in the region of the 

 nucleus (fig. 41). 



