J. R. SHAVER 281 



mitoses opposed to that of the mitochondria. This investigator 

 also claimed that there is a "plasmatic rhythm" in the production 

 of mitochondria, which consists of a disappearance of particles 

 during division phases and a reappearance when the nucleus is 

 reconstituted. 



It may be remarked that the figures given by Gustafson and 

 Lenicque (1952, 1955) for overall numbers of mitochondria are 

 based on the relative mitochondrial densities of sample embryonic 

 sectors. 



70 



5 60 



liJ 

 Q. 



z 

 o 



X 



o 

 o 



50 

 40 



30 



20 

 15 



1 I 



UF 5 10 15 20 25 30 35 40 



HOURS 



Fig. 3. Lytechinus pictus, cultured at 19.5° C. Numbers of mitochon- 

 dria per embryo at different times in early development. 



Shaver (1955, 1956) has attempted to estimate the numbers 

 of mitochondria at different stages in sea urchin development, 

 with a modification of quantitative technique described by Shel- 

 ton et at. (1953) for counting mitochondria in liver homogenates. 



Preliminary studies were made to ascertain optimum conditions 

 of homogenization medium for the preservation of mitochondria 

 (I.IM sucrose made up in 0.0005M phosphate buffer at pH 7.4- 

 7.6), method of homogenization (breaking of eggs or embryos by 

 means of a mechanically operated syringe and No. 20 canula), 



