74 Papers from the Department of Marine Biology. 



significant in these studies. The eggs were shaken and several fields 

 were counted, totaling 100 to 200 or more eggs. At least two counts 

 were made, the first when most of the eggs were in the 2-cell stage — i. e., 

 1 hour — and the second when most eggs were in the 4- or 8-cell stage — 

 i. e., 2 hours. Preliminary experiments showed that there was little or 

 no increase in total cleavage after the second hour in Toxopneustes 

 and Arbacia. Therefore the number of eggs that had cleaved in 2 

 hours was taken as the maximum. In many instances a count was 

 made 40 minutes after fertilization, when the eggs first began to cleave. 

 These three counts afforded the bases for computing the rate of early 

 cleavage and the total cleavage, and also served as a check upon a 

 possible error in the previous count. 



Most of the experiments are referred to by date. Thus 7/5 means 

 that the experiment was made on July 5, 1916. 



CONCENTRATION OF THE GERM-CELLS. 



It was important to determine not only whether differences in con- 

 centration of the germ-cells made for differences in membrane forma- 

 tion, in cleavage, etc., but also to determine what concentration gave 

 optimum results for each species of sea-urchin. This was done in the 

 following preliminary experiments which consisted in varying the quan- 

 tity (1) of eggs of a given female, (2) of sperm, and (3) of sea- water. 

 Tables 1 and 2 give the results of a few such experiments, with so-called 

 "good" and "poor" eggs. 



TOXOPNEUSTES. 



With the volume of sea-water and the concentration of sperm con- 

 stant, and with increasing numbers of eggs, a concentration was finally 

 reached in which the rate of membrane formation, rate of cleavage, and 

 total cleavage was considerably reduced. 



In experiment 7/5 this injurious concentration was 1,084 eggs in 

 10 c.c. of sea-water in a Syracuse dish; 8,800 eggs in 7/8a; 9,120 eggs in 

 7/7a; 21,920 eggs in 7/13a; 38,400 eggs in 7/9a. 



In a few experiments the injury was overcome in part by increasing 

 the volume of sea-water 4 to 80 times; in most experiments the injury 

 was not overcome in this manner. (See 7/7b, 7/8b, 7/13c, 7/9b.) 



The decreased cleavage was due in part to insufficiency of sperm. 

 The experiments to test this are in entire accord with those of LilHe. 

 An increase of sperm increased cleavage 300 to 800 per cent. (See 

 7/8c, 7/9c, 7/13 d and e.) A decrease of sperm with low concentration 

 of eggs did not affect the total cleavage. (See 7/13b.) 



The lowered cleavage in high concentration of eggs was largely due 

 to asphyxiation ; this was evident in the experiments in which the high 

 concentrations of eggs were gently shaken and the cleavage jumped 



