262 FINE-STRUCTURE OF PROTOPLASM II 



arises whether the unit of assimilation is identical with the chloro- 

 plastin unit. This would simplify our terminology. But as long as it 

 cannot be proved that chloroplastin derives from the grana alone, the 

 coincidence of the number of chlorophyll molecules in the unit of 

 assimilation and in the chloroplastin molecule seems to be only 

 incidental. 



§ 4. Erythrocytes 



a. The Microscopic Structure of Erythrocytes 



It is not only their lack of a nucleus which makes the red blood 

 corpuscles of mammals a cytological curiosity, but it is also the 

 peculiar shape of the cell. Seen from the top in the microscope, they 



look like round discs, the bound- 

 ary of the cross-section of which 

 is curiously sinuate, instead of 

 being planoparallel. Thus the 

 erythrocytes are biconcave discs. 



This remarkable shape of the 

 Fig. 1^2. Cross-section of the red cell of man. . . • j j. i j 



,^ „ , . , 1/ / J , rv cross-section is said to be due 



ab = 8.55/i; thickness li (cd 4- ef) = 2.40//; 



thickness gh = 1.02 /i (from Ponder, 1934)- to the function of the red blood 



corpuscles, since from a surface 

 thus shaped the interior of the cell can be easily supplied with oxygen 

 by diffusion, whereas a globular shape would entail greater poverty 

 of oxygen in the centre than in the surface layers and, with a piano- 

 parallel disc, the edge would be richer in oxygen than the centre. 



The discs remain biconvex in shape as long as the erythrocytes are 

 suspended in the blood plasma or in serum, but they round up 

 directly if the medium is changed by the addition of lecithin to the 

 blood plasma. It is a remarkable fact that the same thing happens 

 when a thin layer of them is covered with a cover glass. Ponder 

 (1934), discussing many possible causes of this phenomenon, omits 

 to mention the change in r^ of the medium and asphyxiation, which 

 all living cells undergo after some time in the thin layer under the 

 cover glass. Under certain circumstances rounded blood corpuscles 

 can be restored to their initial biconcave disc shape by the addition 

 of serum. 



As any experiment with erythrocytes involves possible transfor- 

 mation, it is not an easy task to establish their true cross-sectional 



