J. R. SHAVER 283 



the number per cell would be approximately halved at each divi- 

 sion, but the total number of mitochondria per embryo would 

 remain the same. The obvious alternative to this interpretation is 

 that mitochondria are destroyed and produced at about the same 

 rates during this period as Agrell ( 1955 ) suggested, but at pres- 

 ent there seems to be no way of deciding between the two possi- 

 bilities. 



2. In the advanced blastula stage, just prior to the beginning of 

 migration of the primary mesenchyme cells ( 16 hours for L. pic- 

 tiis, 20 hours for S. piirpiirattts), there is a sharp increase in mito- 

 chondrial number. This increase in number is consistent with the 

 suggestion (Gustafson and Lenicque, 1955) that mitochondria 

 do not increase markedly during cleavage, but there is no evi- 

 dence from this study that mitochondria result from an aggrega- 

 tion of smaller precursors. 



3. The level of mitochondria reached at the mesenchyme blas- 

 tula stage is maintained for some hours. At the time when gut 

 differentiation is progressing (about 28 hours in L. pictiis, 33 

 hours in S. purpuratus) there is a sharp decrease in mitochondria, 

 reaching a level below that of the uncleaved egg. 



4. In S. purpuratus, where later developmental stages were 

 studied, mitochondria in pluteus stages have regained the level 

 of the uncleaved egg. 



The figures presented afford only a rough estimate of mitochon- 

 drial numbers during sea urchin development. The method used, 

 however, appears to be more reliable for quantitative purposes 

 than techniques which depend on the identification of particles 

 in intact cells. If it is assumed that the estimations approximate 

 reality, a few questions raised by such fluctuations of mitochon- 

 drial populations in sea urchin development may be noted: 



1. If the numbers of mitochondria do, in fact, remain approxi- 

 mately constant during the cleavage period, what does this mean 

 in relation to the energy-requiring processes occurring at this 

 time? Are we dealing with a situation in which the uncleaved egg 

 has stored enough material, not only to provide for the syntheses 

 which go on during cell division but also to provide for the energy 



