ERYTHROBLASTS IN THE PIG EMBRYO 171 



pleted constriction, subsequently fragmented off, thus leaving an 

 apparently cytoplasm-free nucleus. The occurrence of the so- 

 called free erythrocytic nuclei does not, therefore, appear to be 

 conclusive as regarding either the question of nuclear extrusion or 

 of cytoplasmic constriction. 



The occurrence, however, of similar nuclei plus a small quan- 

 tity of hemoglobin-containing cytoplasm would be evidence of a 

 more decisive character. Figures 10 n, 11 n, 12 n, represent 

 small nucleated corpuscles in the embryonic pig blood which were 

 if anything more frequent than the free erythrocytic nuclei. The 

 cytoplasm, which varies from a narrow rim around the nucleus to 

 a larger quantity as illustrated in the drawings, takes a hemo- 

 globin stain with Giemsa's fluid. The nuclei are small, com- 

 pact, and in stain and structure appear indistinguishable from 

 either the nuclei of the fully differentiated erythroblasts or from 

 the free nuclei. Some of these nuclei also present a flattened 

 form (figs. 11, 12) similar to the flattened nuclei previously de- 

 scribed for the erythroblasts (p. 135). These structural charac- 

 teristics do not favor regarding these cells as young undiffer- 

 entiated erythroblasts. On the contrary, their size, form, nuclear 

 and cytoplasmic structure and stain conform cytologically with 

 the nucleated portions remaining after the completion of cyto- 

 plasmic constrictions in the cultures, and what should be expected 

 in the blood of the embryo if a similar process occurs normally 

 within the embryonic circulation. 



In studying the literature bearing on the present subject it is 

 important to note that while Howell ('91) failed to find in the 

 cat embryo erythrocytic nuclei with a small quantity of cyto- 

 plasm such as just described in the preceding account, on the other 

 hand. Jolly ('07) states his observation that "dans certain cas, 

 comme chez le rat, le pore, sur beaucoup de globules, le disco- 

 plasme apparatt comme reduit, si bien qu' on pourrait se demander 

 si on a affaire a des noyaux expulses avec une certaine quantity 

 de protoplasme hemoglobique " (p. 207). 



As for the fate of these nucleated remainders the data at hand 

 does not justify a conclusion as to whether some of them, at least 

 such as may have an appreciable quantity of cytoplasm, may 



