336 Prof. E. W. MacBride. The Artificial Production of 



amount of all the salts present in the water to 37 parts per thousand. 

 When the method of evaporation is used, the -concentration of all the salts 

 present in sea-water is raised in equal proportions, and, in addition, the 

 acidity of water is reduced by the expulsion of CO2. In the method 

 employed in 1917, the acidity of the water was undisturbed, and the 

 concentration of NaCl alone was raised. 



The eggs were fertilised on July 7, and on July 11 some of the larvae 

 were transferred to Breffit jars with hypertonic water ; other larvse were 

 retained in normal sea-water to serve as controls. 



On July 17, when the larvae had lived for six days in the hypertonic 

 water, a large number of them were transferred to plunger jars. At this 

 time they were showing signs of the division of the coelomic sacs into 

 anterior and posterior portions (fig. 2). 



I had altogether eight plunger jars at my disposal — four being situated on 

 each side of the tank-room. To show how slight are the variations in con- 

 ditions of light, etc., which make for success or failure, I may mention that 

 only the cultures on one side of the tank-room were successful, and we shall 

 therefore confine our attention to this group of four jars. Of these, one was 

 filled with hypertonic water and the Nitzschia would not flourish in it, and 

 as a consequence all the larvae perished. The remaining three jars were 

 filled with normal sea-water, and each received the contents of a Breffit jar 

 which must have contained over a thousand larvae ; two of the plunger jars 

 received the larvae from " salted " Breffit jars, and one those from a Breffit 

 jar which had contained normal sea-water. The culture in this jar, therefore, 

 served as a control with which to compare the results of the cultures in the 

 other two. It is necessary to add that all the larvae which were added to 

 the plunger jars were the offspring of the same cross. 



This method of starting a culture of Echinoderm larvae in a plunger jar 

 differs from the method which I adopted in previous attempts to rear the 

 larvae of Echinus esculentus and Echinus miliaris. My former method was 

 to pick out about 300 of the most healthy looking larvae from a Breffit jar 

 and transfer them to a plunger jar. This method sometimes succeeded, but 

 often failed — in fact one may say that it always failed unless the larvas were 

 taken from the Breffit jar when they had attained a relatively advanced 

 state of development, when, in fact, all their weaker brethren had perished. 



The reason for this seems to be that the eye of the experimenter is unable 

 to select the really healthy larvae, and that the larvae must therefore be 

 subjected to a period of struggle — in a word of natural selection — in order 

 that those individuals may be found which are sufficiently vigorous to be able 

 to survive until they complete their metamorphosis. 



