III. 8 CHEMICAL COMPOSITION 151 



but motionless, and doomed to eventual degeneration and death 

 (Fig. 74 A, a, b). Very occasionally a gastrula with a short gut 

 is formed. 



When the blastulae are immersed in the solution they give 

 rise to small, opaque gastrulae. 



A certain degree of alkalinity is necessary for fertilization. 

 The spermatozoa are less sensitive to a want of alkalinity, more 

 sensitive to excessive alkalinity than the ova. 



By the addition of a small amount of sodium hydrate to the 

 neutral medium development is accelerated, but an increase of 

 the alkalinity of ordinary sea-water is unfavourable. Loeb, on 

 the other hand, has found that the addition of from -006 % to 

 008 % sodium hydrate to sea- water accelerates the development 

 of Arlacia. 



The formation of pigment and the vibration of the cilia are 

 other processes which depend on the presence of the hydroxyl 

 ion. Plutei die without it and their skeleton is dissolved. 



The function of the ion does not appear to be to neutralize 

 any acids produced by the tissues, for these give a neutral 

 reaction even in OH-free media. 



Since aeration improves the development of eggs in these 

 media, and the more so if the air is deprived of its carbon dioxide, 

 Herbst has concluded that one function of the OH ion is to 

 neutralize the CO 2 and allow of the formation of the necessary 

 carbonates. Another function is possibly, as Loeb suggested, 

 the acceleration of processes of oxidation. 



The experiments which we have been considering are unique 

 of their kind, and it is impossible to exaggerate their importance. 

 For, whatever may be the ultimate explanation of the facts, there 

 can be no doubt whatever that the most complete demonstration 

 has been given of the absolute necessity of many of the elements 

 occurring in ordinary sea-water, its normal environment, for the 

 proper growth and differentiation of the larva of the sea-urchin. 

 Nor is this all. Some of the substances are necessary for one 

 part or phase of development, some for another, some from the 

 very beginning, others only later on. Thus potassium, mag- 

 nesium, and a certain degree of alkalinity are essential for 



