282 



EARLY ECHINODERM DEVELOPMENT 



dilution of sample for counting, etc. Counts were made in a 

 Petroff-Hauser bacterial counting chamber, at a magnification of 

 1400X, with a dark M phase contrast oil immersion objective. 

 The criteria used for identification of the mitochondria in homo- 

 genates included size, structure, intensity of contrast, behavior 

 in media of different osmotic concentrations, and the ability to 

 undergo changes in shape (cf. Harman, 1950). 



The results are presented in Figs. 3 and 4. They are plotted as 

 confidence intervals calculated from the raw data at the 90% 



3 



O 



>- 

 cr 



CD 



< 

 cr 

 o 



z 

 o 



X 



o 

 o 



UF 5 10 15 20 25 30 35—50 60 70 80 

 HOURS 



Fig. 4. Strongijlocentrotiis purpuratus, cultured at 17° C. Numbers of 

 mitochondria per embryo at different times in early development. 



level, with a statistical treatment the details of which will be 

 published elsewhere. 



Several points may be briefly noted in connection with these 

 results : 



1. In both species studied, the number of mitochondria re- 

 mains constant during the first twelve or fifteen hours of devel- 

 opment ( temperature of culturing and rates of development were 

 different for each species). This result indicates that mitochon- 

 dria do not multiply during cleavage, but are apportioned among 

 the dividing cells. If equal distribution of the particles is assumed, 



