I CYTOPLASM 137 



in KCl, whereas the second show a decrease. With this test Ranzi has 

 shown that the euglobulin of frog embryos is fibrillar between p^ 

 5.5 and 8.5; but beyond this range it is globular. Since the develop- 

 ment of the frog ectoderm in tissue cultures is only possible within 

 the range of pH 5 and 9, there is an indication that fibrillar proteins 

 are indispensable for any manifestation of morphogenesis. 



Lipids. The biological concept of lipids comprises all substances 

 which are hydrophobic. This concept is therefore characterized by a 

 negative property (insolubility in water) rather than by a positive one 



:y OH-CH2 y CH3 



-c I ~^ 



§■ OH-CH "I CH3 



■0 I Q. 



1^ OH-CH2 -■ CH.f 



3 Fatty acids + Glycerol 







Lipophilic CH3 \y\y\/sy\y\/\/\/\ C-O-CH, /\/\/vv/VN/s,/\/sy\/v/\/'VN/ CW-, Lipophilic 



Falty acid Higher alcohol 



Wax 



Fig. 91. Molecular structure of lipid chains. 



(solubility in organic liquids). For this reason it comprises different 

 families of substances such as terpenes, waxes, fats, sterines, etc., of 

 which the last two take part in building up protoplasm. 



True lipids are characterized by the fact that all their free end groups 

 consist of typically Hpophilic groups. This is especially obvious in the 

 case of fats, which represent esters of the three-valent alcohol glycerol 

 with fatty or oleic acids. As a result of the esterification, thehydrophilic 

 groups of the original products are screened, as shown in Fig. 91. In 

 the same way the hydrophilic groups are masked in waxes which are 

 formed by the esterification of higher alcohols with higher fatty 

 acids (Table XXVII, p. 296). It is difficult to say why they are screened 

 in the course of the metabolic process, but in any case these lipids 

 contrast strongly with the hydrophilic compounds of living cytoplasm 

 and, if they are formed in excess, we observe the well-known phenom- 

 enon of fatty degeneration of protoplasm. A correct balance between 

 hydrophilic and Hpophilic compounds in living matter is essential. 



In contrast to fats, most lipophilic compounds of the cytoplasm 

 carry at least one hydrophiUc group, which serves to bring about the 



