318 GRACE MEDES 



1. Daily fluctuations in the sea-water in respect to 



a. Density 



b. Temperature 



2. Variations in the germinal products at different seasons 



B. Factors that could be controlled 



1. Variations in the acidity and the alkalinity of the medium 



2. Variations in the constitution of the medium 



a. Accompanied by changes in density 



b. Unaccompanied by changes in density 



3. Presence of the products of metabolism 



C. Combined effects of two or more of the factors in 1 and 2 



D. Probable range of variation in Arbacia larvae in their natural 



environment 

 VI. Summary 430 



INTRODUCTION 



The investigation presented in this paper was undertaken 

 for the purpose of determining the normal development of the 

 larvae of Arbacia punctulata under standard laboratory condi- 

 tions, the extent and causes of variation which they exhibit, and 

 the probable range of variation which they may undergo in 

 their natural environment. 



Much of the information available as to normal development 

 during the period usually employed in experimental work must 

 be gathered from control cultures accompanying the various 

 lines of experimentation. These studies, however, most often 

 deal with some limited phase of growth, such as fertilization or 

 cleavage and hence offer but fragmentary accounts of normal 

 development. It has seemed desirable, therefore, to obtain 

 more detailed knowledge than is at present recorded as to de- 

 velopment under laboratory conditions. 



To accomplish this, two lines of experimentation were pur- 

 sued: first, the observation of control cultures raised under 

 conditions as nearly constant as possible; and second, the modi- 

 fication of the medium by varying the individual factors which 

 constitute it. Through the first line of experimentation, it was 

 hoped, not only to obtain a standard type that might serve as a 

 basis for future experimental work, but also to determine the 

 extent and causes of such variations as occur under constant 

 conditions of the surrounding medium. Through the second line 



