FERTILIZATION 205 



differentiation, occurring subsequently to maturation. Cer- 

 tain acids and some other substances seem to have, accord- 

 ing to Morse, a specific effect in bringing about maturation, 

 either not producing cleavage or actually inhibiting it, in 

 which latter case it may then be induced by other treatment. 

 The precise actions upon the egg of those chemicals inaugu- 

 rating cleavage are varied, but for the most part they appear 

 to effect certain changes in the egg which are similar in nearly 

 every instance. For example, according to Loeb, who is the 

 pioneer in this important work, in the sea-urchin, the best 

 result, that is, the closest imitation of natural fertilization, is 

 secured by treating the eggs, first with a solution of a monobasic 

 fatty acid, such as butyric acid, for one or two minutes. In 

 many cases the butyric acid can be replaced by an alkaline 

 solution of equivalent strength or by a solution of almost any 

 fat solvent. This treatment results in the formation of an 

 apparently typical fertilization membrane. Then second, the 

 eggs are treated for some minutes or hours (sea-urchin eggs, 

 thirty to fifty minutes at 15 C.) with a hypertonic sea water, 

 that is, sea water whose osmotic pressure has been raised about 

 50 per cent, above normal by the addition of salts, such as 

 sodium chloride. Finally the eggs are returned to normal sea 

 water and cleavage then follows in quite the usual fashion. 



What is actually accomplished within the egg by such treat- 

 ment as this is largely conjectural. Loeb suggests that the 

 process may be as follows. The mature ovum is surrounded by 

 a relatively impermeable surface film which prevents the 

 oxidations necessary to development. The butyric acid or 

 similarly acting substance, by dissolving certain fatty constitu- 

 ents near the egg surface, frees from association with these, 

 certain other osmotically active materials which then form the 

 permeable fertilization membrane, and thus rapid oxidations 

 are permitted. Loeb believes that the nuclear substance 

 possesses a catalyzer w r hich, in the presence of oxygen, brings 

 about a synthesis of nuclein, one of the chief constituents of 

 chromatin, and that this synthesis of nuclein is the chief 

 chemical action of the segmenting ovum. This process of 



