J. R. SHAVER 285 



ceeded in numbers by smaller rod-shaped forms in the gastrula 

 and later stages. 



Caspari (1955) suggested that genetically controlled differ- 

 ences exist in mitochondria of the cells of the same tissues in 

 different strains of mice. From his study there was also some evi- 

 dence of tissue differences in mitochondria in the same animal. 

 The idea is advanced that the composition of particle types in a 

 particular kind of cell is an expression of its developmental and 

 physiological activities. It would be interesting to test this hy- 

 pothesis by studying mitochondrial heterogeneity in the cells of 

 developing embryos, but echinoderm material evidently would 

 not be appropriate, owing to the lack of genetic information a1)0ut 

 this group of animals. 



Summary 



A brief review is given of observations and experiments dealing 

 with cytoplasmic particles in development. Some hypotheses con- 

 cerning their role in differentiation are discussed. The results of 

 some current work by the author on changes in number and dis- 

 tribution of mitochondria during development of the sea urchin 

 are presented. Marked changes in number and, possibly, kinds of 

 mitochondria are evident during development. Especially notice- 

 able is an increase in numbers of the particles in mesenchyme 

 blastulae and gastrulae, followed by a sharp decrease in immedi- 

 ately subsequent stages. A gradual return to the earlier level of 

 mitochondrial density was obsei"ved in one of the two species 

 studied. The author has failed to find, in the material studied by 

 him, that mitochondria are distributed along a gradient in mesen- 

 chyme blastulae and gastrulae, as has been reported in another 

 sea urchin species. This result, therefore, does not support hy- 

 potheses of differentiation based on such a distribution pattern of 

 these particles. Technical details are given concerning the iden- 

 tification of mitochondria in intact sea urchin embryos, in living 

 and fixed condition, as well as in homogenates. 



