ERYTHROBLASTS IN THE PIG EMBRYO 177 



tial evidence has so far been obtained indicating the correctness 

 of such a distinction and it has been assumed that these large red 

 blood cells are not fundamentally different from normoblasts in 

 their mode of differentiation, both types giving rise to non-nucle- 

 ated erythrocytes. It is important to note that the more recent 

 investigators arrive at a similar conclusion for mammals in general. 

 Weidenreich ('05) describes megaloblasts and normoblasts of 

 the embryo as differing merely in the size of the cells, rather than 

 in anything more fundamental in character, and concludes that 



Es besteht keine Trennung der Form en in Erlichschen sinne (p. 434). 

 Der fotale Erythrozyt variiert in seiner Grosse betrachlicher als der 

 postfotale; neben ganz grossen Formen (Gigantozyten) fin den sich in 

 fotalen Leben grosse (Makrozyten) , normale (Normozyten) und auf- 

 fallend kleine (Mikrozyten), die aber alle durch tJbergange verbunden 

 erscheinen. Das gleiche gilt aber auch f iir das kernhaltige Stadium ; es gibt 

 Gigantoblasten, Megaloblasten, Normoblasten und Mikroblasten und 

 dasz^vischen gleichfalls alle Ubergiinge. Streng genommen miisste man 

 also alle cUese Formen als besondere Typen auffassen (p. 449). 



Jolly ('07) fails to find sufficient ground for Erlich's distinction; 

 ''nous pensons que cette opinion resulte surtout d' une confusion, 

 et que, en voulant faire, avec Erlich, deux especes speciales des 

 normoblastes et des megaloblastes, on risquerait de se tromper" 

 (p. 276). Minot ('12) reaches a similar conclusion, as indicated 

 in the folowing reference to Erlich's classification. 



Weidenreich expresses himself positively against this opinion, justly 

 it seems to me. In fact, the mesamoeboids in young embryos vary much 

 in size (compare fig. 354) and a similar unevenness prevails also among 

 the ichthyoid cells (fig. 357), It is further probable that the large 

 mesamoeboids, of which the majority form small cells by continual divi- 

 sion, in small part at least develop haemoglobin precociously and thus 

 produce the so-called megaloblasts (p. 507). 



2. EVIDENCE FROM PREVIOUS INVESTIGATORS BEARING ON THE 

 ORIGIN OF PLASTIDS BY CYTOPLASMIC CONSTRICTION 



a. In mammals 



After having made the observations described in the present 

 paper indicating the origin of plastids by cytoplasmic constriction, 

 it was later found in an examination of the literature or erythro- 

 cytes that the possibility of such an origin for these red corpuscles 



