ERYTHROB LASTS IN THE PIG EMBRYO 153 



b. General characteristics of the erythrocytes as first observed in 



in the cultures 



In the examination of the preparations when first made, the 

 various types of red blood cells found present consist of the large 

 nucleated erythroblasts with more or less eccentrically located 

 nuclei, non-nucleated plastids varying in size, smaller erythro- 

 blasts with a narrow rim of hemoglobin containing cytoplasm, and 

 nuclei apparently free from cytoplasm; the relative proportion of 

 nucleated and non-nucleated elements depends upon the stage of 

 differentiation of the blood used for the cultures, which it may 

 be noted, is by no means always the same for embryos of equal 

 size but taken from different uteri. The cytoplasm of both nu- 

 cleated and non-nucleated elements is apparently homogeneous in 

 structure and presents a more or less yellowish color, depending 

 upon the degree of homoglobin differentiation. The majority of 

 erythrocytes present an even rounded contour. Some of the cells, 

 however, may be elongated in a pear-shaped manner with the 

 nucleus at the smaller end, and in others the cytoplasm may ex- 

 tend out in blunt, rounded, somewhat bud-like projection. Oc- 

 casional nucleated as well as non-nucleated erythrocytes may 

 possess slender, pointed cytoplasmic processes, the probable 

 nature of which has already been considered (pp. 138-140). In 

 most instances the cytoplasm of the cells, aside from certain 

 form changes to be described presently, is more or less quiescent 

 with the exception of the slender pointed processes just mentioned, 

 which may manifest quivering and even oscillatory movements. 

 In the case of the smaller younger erythroblasts where the greater 

 part of the cell may undergo slow changes in form, as may be 

 observed directly in the vessels (fig. 36), these cytoplasmic move- 

 ments are probably not inadequately described as amoeboid in 

 character. Whether the same conclusion is to be drawn for the 

 oscillatory movements of the slender processes of the more mature 

 erythroblasts is less certain, especially as processes with similar 

 activities may occasionally occur in the non-nucleated erythro- 

 cytes.^ 



' In regard to this question of amoeboid motion, it may be questioned whether 

 our present knowledge is as conclusive as cuold be desired as to just when the 



