ERYTHROBLASTS IN THE PIG EMBRYO 167 



tions examined at once. The amnion and its vessels (the later 

 probably chorionic in origin) proved most suitable on account of 

 its transparency. By opening the amniotic sac (while the heart 

 of the embryo was of course still beating), gently arranging the 

 membrane upon a slide and covering lightly with a cover-glass, 

 fairly good views could be obtained of the blood corpuscles glid- 

 ing along through the vessels and capillaries. Their constant 

 motion rendered it difficult to obtain accurate camera drawings 

 but the nature of the observations is indicated in figures 32 and 33 

 from a 22 mm. embryo. Figure 32 shows an erythroblast lying 

 within a medium sized vessel, the walls of which are indicated in 

 outline. It will be observed that the erythroblast drawn has 

 become almost completely sub-divided into two parts, the one 

 nucleated, the other non-nucleated. It was not possible to follow 

 this cell long enough to ascertain the completion of the process. 

 In the case illustrated in figure 33, in which the erythroblast came 

 under observation just before a constriction occurred, circum- 

 stances fortunately favored the continuation of the observation 

 for several minutes. WTien first seen at 4.02 p. m.the cytoplasmic 

 pole of this cell manifested activities tending toward a subdivision 

 of its cytoplasm, to all appearances comparable in every respect 

 with the constriction behavior already described in the cultures. 

 Four minutes later the constriction was completed and hemo- 

 globin containing plastid had become entirely separated from the 

 parent erythroblast. 



b. In fixed vessels 



Figures 5 and 6 are camera drawings of erythroblasts found 

 in vessels which had been carefully fixed with Zenker-formalin at 

 a temperature of about 38°. The vessels selected for study were 

 those occurring in the membranes surrounding the amniotic fluid. 

 These membranes were fixed in situ before removal from the 

 uterus, then stained, cleared, and suitable pieces mounted in toto. 

 If one examines carefully the nucleated erythrocytes within these 

 vessels cells are occasionally found having the characteristics indi- 

 cated in the figures, in which there is a decided constriction of the 

 cell body in the region between the nuclear and cytoplasmic poles. 



