320 DEVELOPMENT OF ASCIDIANS 



muscular contraction, cellular secretion, and synthesis of biologi- 

 cal substances. 



The embryologists, always in search of that "something" re- 

 sponsible for cellular and organic differentiation, felt immediately 

 that they would be greatly helped by the new biochemical re- 

 search: the factors responsible for differentiation and induction 

 must have a chemical basis! Particularly interesting to them was 

 the fact that the mitochondria contain dozens or hundreds of 

 enzymes. These are not distributed at random but, on the con- 

 trary, are assembled in a fixed pattern, that is, locally disposed, 

 spatially connected, and in determined quantities. 



It is a well-established fact in genetics that the chromosomes 

 also possess a preordered and constant pattern, and there are 

 valid cases in which results have sufficiently demonstrated that 

 genes ai-e enzymes, or at least produce enzymes. Another con- 

 sideration associates in the mind of an embryologist the mitochon- 

 dria and the chromosomes: the fact that, as in the case of the 

 chromosomes, the mitochondria can be separated, with respect 

 to their form and functions, into many different classes. Generally 

 it is assumed that every mitochondrion possesses the power of 

 operating the Krebs cycle, the transfer of electrons to molecular 

 oxygen and the oxidative phosphorylation; but other functions 

 are typical of certain classes of mitochondria. There have been 

 described (Lenicque, 1953) two kinds of mitochondria in the 

 muscles of amphibians, one devoted to the lipidic metabolism, the 

 other implicated in the energetics of contraction. In the liver, 

 too, there are known to exist two kinds of mitochondria which 

 provide two different functions, the synthesis of citrulline and 

 the detoxication of aromatic substances. The brain mitochondria 

 are not able to catalyze the reactions of fat metabolism, and the 

 maximum of differentiation and specialization is represented by 

 the mitochondria of the secretory cells, which perform the most 

 diverse functions. Very often one speaks of "ectodermic" or "en- 

 todermic" mitochondria to indicate theii- principal role. The 

 former would carry out the synthesis of some proteins, and the 

 latter the synthesis of pigments and other substances. In the 



