ERYTHROBLASTS IN THE PIG EMBRYO 143 



fully differentiated erythroblast, a distinguishing feature of which 

 would appear to consist in a predominance of a stroma structure 

 at the nuclear pole, whereas the cytoplasmic pole of the disc, 

 deficient in stroma, is composed of a greater amount of fluid or 

 semifluid hemoglobin containing substance; (see also further dis- 

 cussion, pp. 147-148). 



5. DISCUSSION WITH REFERENCE TO CERTAIN PROBLEMS IN THE 

 CYTOMORPHOSIS OF THE MAMMALIAN ERYTHROCYTE 



a. The question of the correlation of the form of the definitive plastid 

 with nuclear extrusion 



Concerning the origin of the biconcave disc form of the mam- 

 malian erythrocyte, Howell ('91) in a summation of the evidence 

 for nuclear extrusion concludes that 'Hhe biconcavity of the 

 red blood corpuscle is probably caused in the first place by the 

 removal of the nucleus from the middle of the spherical cell" 

 (p. 113). Regarding the same subject Maximow ('09) states for 

 the rabbit embryo "Als Resultat dieses Kernaustrittes finden wir 

 dann nackte, in dem Blutplasma frei flottierende kerne und kern- 

 lose rote Blutkorperchen von mehr oder weniger regelmassiger 

 Scheiben-oder Glockenform" (p. 485). Wie4enreich ('05, p. 445), 

 in a discussion of the assumption of a cup form on the basis of the 

 passage of a quantity of substance from the cell, notes ''dass die 

 Tatsache, dass die Entstehung der Napfform mit dem Kernverlust 

 zusammenfallt, einen Beweis mehr fiir die Lehre vom Kernanstritt 

 liefert;" (cf. also Weidenreich '10, p. 319). 



These references are sufficient to make evident the tendency to 

 explain the disc form of the definitive corpuscle as a result, in part 

 at least, of the extrusion of the nucleus from the cell. In drawing 

 this conclusion the erythroblast, previous to denucleation, is of 

 course assumed to be spherical in form. But the observations 

 considered in the present paper offer evidence that this assump- 

 tion does not always hold, for it appears that in the pig embryo the 

 erythroblasts may become disc or cup shaped while still retaining 

 their nuclei. A possible explanation which has been suggested 

 for these facts may be incidentally noted. Occasionally erythro- 



THE AMERICAN JOURXAL OF AXATOMY, VOL. 16, NO. 2 



