284 EARLY ECHINODERM DEVELOPMENT 



demands of the system? Or, as has been suggested, are mitochon- 

 dria destroyed and produced at a rate corresponding to a plas- 

 matic rhythm (Agrell, 1955)? 



2. Does the apparent increase in mitochondria in the mesen- 

 chyme blastula reflect new demands of the embryonic system in 

 preparation for, or coincident with, the beginning of primary dif- 

 ferentiation? This question is loaded with imponderables, since 

 we as yet seem to be quite unaware of any specific distinctions 

 that can be made among the demands of the embryonic cell for 

 growth, maintenance, or transformation. To ask whether or not 

 the increase in mitochondria reflects the requirements of the em- 

 bryo at this time for new protein types seems merely to be 

 rephrasing the question. 



3. What is the meaning of the decrease in mitochondrial num- 

 ber, shortly after the peak reached at the beginning of gastrula- 

 tion? It has been reported by Gustafson and Lenicque (1952) 

 and Shaver ( 1955 ) that, in intact stained embryos, after gut dif- 

 ferentiation has progressed to some extent, there is a diminution 

 in mitochondrial number in the cells of the body wall and a con- 

 centration of particles in the cells of gut structures and of ciliated 

 bands. An obvious guess as to the nature of the decrease in parti- 

 cles at this time is that synthetic processes or physiological activ- 

 ity, or both, have become restricted to a smaller area of the em- 

 bryo, producing an overall diminution in mitochondria. 



4. In addition to questions raised by changes in total numbers 

 of mitochondria, the problem arises of changes in classes of mito- 

 chondria during early development. Recent studies by Paigen 

 (1954) and Novikoff et al. (1953), and others, of mitochondrial 

 heterogeneity, indicate that in adult mammalian cells the chemi- 

 cal structure (and function?) of mitochondria varies with their 

 size. Considering the range of dimensions of the particles counted 

 in this study, it is possible that transitions occur from one size 

 class to another which would be revealed only by differential 

 counts. Although no quantitative data are available, the impres- 

 sion was gained by Shaver (1955) that the larger spherical or 

 oval forms seen in the uncleaved egg and earlier stages were ex- 



