FATS 



467 



in lipoids. Lecithin is rather strongly hydrophilic; it takes up 

 water readily and emulsifies quickly; it may, therefore, function 

 in determining the permeability of cells (page 289). It is also 

 possible that fatlike substances play their part in determining 

 cell permeability not because of the emulsions they form but 

 because of the orientation of their distinctly linear and often 

 polar molecules. (This important problem is discussed in 

 detail on page 256.) That fat molecules may be linear is indi- 



FiG. 175. — Protoplasmic processes formed at the surface of unripe Echinoderm 



eggs in the presence of sperm. 



cated by the molecule of squalene (in this case a nonpolar one), 

 discovered by Tsujimoto. It is the chief constituent of the liver 

 oils of elasmobranch fish and has been synthesized by Karrer 

 thus establishing the following formula: 



CH3 CH3 CH3 CH3 CH3 CH3 



|HHH|HHH|HHHH|HHH|HHH| 



C:CCC-C:CCC-C:CCCC:CCCC:CCC-C:C 



I HH HH HH HH HH I 



CH3 CH3 



Another Ukely role of lecithin is its part in the formation of 

 protoplasmic processes, such as surface protrusions, papillae, 

 and small pseudopodia. Whiplike structures, which may be 

 unusual and useless or common and func- 

 tional, often appear on the surface of proto- 

 plasm. Certain of these (Fig. 175) may be 

 identical with the myelin forms produced 

 by lecithin when in contact with water, 

 alkalies, acids, etc. These surface out- 



,1 r 1- -J £ i J -u A Fig. 176. — Myelin proc- 



growths from lipoids were first described ^gg^^ ^^^^^^ by lecithin 

 by Virchow and have more recently been in contact with acid, 

 carefully studied by Leathes (Fig. 176). ^°^ 

 The formation of myelin forms appears to rest upon the hydro- 

 phiHc qualities of lecithin. The phosphatide molecule becomes 

 hydrated, producing a low interfacial tension between it and 



c' 5~ c-rcn-:- -:s.3'«-/TW^(!r.'^!«S^-^ 



