ERYTHROBLASTS IN THE PIG EMBRYO 175 



and fresh material. Turning to the data in the mammaUan 

 embryo bearing on this subject it appears that conclusions for 

 nuclear extrusion are not so self-evident as might be anticipated. 



The first erythroblasts of the embryonic circulation are, as is 

 well known, much larger than those found in both later embryonic 

 life and the adult. The direct evidence for the mode of enuclea- 

 tion of these large erythroblasts, or megaloblasts as they have 

 been designated by Erlich, appears to be inconclusive and has 

 given rise to a diversity of opinion. Erlich (Erlich and Lazarus 

 '09, p. 74) holds that the nuclei of these megaloblasts are dissolved 

 within the cell in contrast to the so-called normoblasts in which 

 he supposed nuclear extrusion to obtain. Weidenreich ('05) on 

 the contrary questions the validity of this conclusion and main- 

 tains that the nuclei are extruded from the erythroblasts of the 

 embryo as well as from those of the adult (p. 434; of. also Weiden- 

 reich '11, p. 45). Jolly ('07) was unable to find any stages in the 

 expulsion of the nuclei of these embryonic cells in either the cir- 

 culation or the liver of the embryo and concludes that the nuclei 

 probably disappear chiefly by atrophy within the cell (p. 278), 

 although at the same time he holds that in the adult the process 

 of enucleation is one of extrusion. Maximow ('09, p. 478) states 

 that the early red blood cells in the rabbit embryo manifest little 

 tendency toward enucleation although he reports some evidence 

 for extrusion in the case of the embryo guinea pig. 



The direct evidence for nuclear extrusion in the embryo thus 

 appears inconclusive, if not contradictory, indeed to such a de- 

 gree that several investigators have even raised the question 

 whether the erythroblasts in the young embryo ever differentiate 

 into non-nucleated corpuscles. However, it must be emphasized 

 that it does not seem that there can be any reasonable doubt, at 

 least for the pig embryo, of the occurrence of large non-nucleated 

 erythrocytes which could have arisen only from the large nucle- 

 ated red corpuscles or megaloblasts. It will be observed that the 

 same fact has also been recognized in the preceding references 

 from both Jolly and Weidenreich (p. 65). On the other hand^ 

 the problem of just how these plastids are derived from the mega- 

 loblasts appears still to be an open one, concerning which the 



THE AMERICAN JOURNAL OF ANATOMY VOL. 16, NO. 2 



