276 EARLY ECHINODERM DEVELOPMENT 



3. Since Gustafson and Lenicque ( 1952 ) report mitochondrial 

 counts made only within the area of a reticulum, the process of 

 flattening embryos for observation might have affected particle 

 distribution within this area. Shaver ( 1955 ) has observed em- 

 bryos during the flattening process and has noted differences in 

 resistance to pressure of cells in animal and vegetal regions, 

 which result in distortion of cell shape and of the distribution of 

 particles within the cytoplasm. 



4. In the fixed and stained preparations, in which mitochondria 

 are much more easily identified than in vitally stained cells, care- 

 ful observation under oil immersion lens revealed no consistent 

 differences in the number of particles in cells along the length of 

 the animal-vegetal axis, in mesenchyme blastulae, or young gas- 

 trulae. Quantitative counts of the particles were not replicable 

 from one embryo to another, however ( Shaver, 1955 ) . 



The inability to distinguish a gradient of distribution of mito- 

 chondrial by Shaver ( 1955 ) indicates either that there are species 

 differences among sea urchins in this respect or that more accu- 

 rate methods of determining the localization of mitochondria 

 should be utilized. Results of electron microscopy of ultra-thin 

 sections of both plant and animal tissues have pointed to a basic 

 similarity of structure of mitochondria from diverse sources Pa- 

 lade, 1952 ) . An application of these techniques to sea urchin em- 

 bryonic material would seem to be a promising approach to the 

 clarification of the problem of particle localization. 



Some other observations by Shaver ( 1955 ) confirm and extend 

 Gustafson and Lenicque's (1952) findings. An examination of 

 early blastula cells, which develop ciliated borders, revealed 

 larger numbers of particles in peripheral regions than in more 

 internal ones. This orientation of particles in ciliated cells was 

 also noted in later larvae stages, when the ciliated bands make 

 their appearance. As development continues ( gastrula and prism 

 larvae), there is a pronounced increase on the number of mito- 

 chondria in the cells of the developing gut structures. In the plu- 

 teus, there are larger numbers of mitochondria in the cells of the 

 developing arms, while the cells of the body wall show a relative 

 decrease in this respect. 



