4 ERYTHROCYTES 27I 



perceptible at all. The probable retardation F can be calculated with 

 the aid of the formula on p. 86 by inserting the value o.oi i for the 

 birefringence An, which Bear and Schmitt (1936) set down for 

 orientated lipid in the nerve sheath. In rabbits, the diameter d^ of the, 

 supposedly, hollow cylindrical rim of erythrocytes is i.j /n (cf. c-d in 

 Fig. 132, p. 262), and d, is shorter by twice the thickness of the bi- 

 molecular Hpid layer (4 X 3 m//), i.e., 1.688 //. The value for the 

 retardation F is then a httle above 1.8 m^. This is a value which, 

 though at the lower limit of quantitative mensurability with sensitive 

 compensators, may, by suitable polarizing optics, be revealed quali- 

 tatively. This shows that a single bimolecular lipid layer suffices to 

 produce the faint positive intrinsic birefringence detected by Schmitt, 

 Bear and Ponder. 



Both the quantity of lipid present and the slight intrinsic bire- 

 frino-ence witness to the fact that there can hardly be more than a 

 double film of orientated lipid molecules in the erythrocyte. This 

 eliminates the possibility of a protein-hpid layer composite body, 

 such as demonstrated in chloroplasts. To account for the lamellar 

 birefringence, therefore, one is forced to assume that the stromatin 

 protein is lamellar with, maybe, layers of water in between. These 

 need not necessarily be continuous; indeed, they are more probably 

 cavities shaped somewhat Hke lenses (Fig. 136, p. 272). On this as- 

 sumption the direction in which the stromatin molecules of Fig. 135 

 (p. 267) (layer IV) are orientated must undoubtedly be turned through 

 an angle of 90° and lie parallel to the erythrocyte surface. 



MiTCHisoN (1950b) is of opinion that the small amount of hpids 

 cannot contribute anything to the birefringence of the erythrocyte. 

 According to him, the birefringence of a bimolecular lipid layer 6 m/t 

 thick is not measurable, due to diffraction errors. He attributes both 

 the negative form and the positive intrinsic double refraction to the 

 stromatin by assuming that radially oriented looped polypeptide 

 chains are lodged in the submicroscopic stromatin layers. Such an 

 arrangement seems to be rather unlikely. 



Electron microscopy. Apart from fibres and diatoms, erythrocytes 

 were the first cytological object to produce good and impressive 

 images in the electron microscope (Wolpers, 1941). This is due to 

 their ability to withstand complete drying without any essential change 

 in structure. 



