HELEN DEAN KING. 



A second experiment was made with leurin on July 24, 1909. 

 In this instance a solution of the strength of % per cent, was 

 employed, since stronger and weaker solutions do not alter the 

 course of development. From the beginning of the experiment 

 the segmentation of these eggs lagged behind that of the eggs in 

 the control lot, and the retardation in development was fully 

 as great as that produced by the I per cent, solution of leucin in 

 the former series of experiments. Later the development of these 

 eggs progressed at a more normal rate, and after seven hours 

 the embryos appeared nearly as well developed, and fully as 

 vigorous, as those in the control. The next morning larvae were 

 swimming at the surface in both cultures, but those in the leucin 

 solution soon dropped to the bottom of the dish and began to 

 disintegrate. Microscopic preparations showed that the very 

 great majority of these larvae were normal in every respect. 



Mathews states that in the summer of 1908, when his experi- 

 ments were made, the sea-urchin eggs showed in many instances 

 the remarkable peculiarity, recorded by Mathews and YVhitcher 

 ('03), that "a large number of eggs while living for several days 

 not forming plutei, or but a small per cent, of irregular plutei." 

 The experiments which Mathews made to test the action of 

 amido-acids on the development of the eggs of Arbacia were made 

 therefore, wholly or in great part, on eggs that were in a peculiar 

 physiological condition when experimented upon: whether they 

 could be considered as normal is doubtful. The unusual types 

 of larva? that Mathews obtained by treating eggs with a weak 

 solution of impure leucin were probably due to abnormal or 

 unusual conditions existing in the eggs at the time of their 

 fertilization, and not to the specific actions of leucin in changing 

 the course of development. The effects of leucin on eggs of 

 Arbacia that are in a normal physiological condition when ferti- 

 lized depends chiefly upon the strength of the solution used: a 

 strong solution retards development and causes the early death 

 of the embryos; a weak solution permits of normal development 

 at first and is toxic only after many hours. 



Tyrosin (CgHnNOs). This substance is not very soluble in cold 

 sea-water, and in order to obtain a solution of sufficient strength 

 one gram of tyrosin crystals was put into looc.c. of sea-water and 



