ECHINODERMS 



91 



even if they originally contain no mitochondria; but it is found that 

 mitochondria gradually appear. It seems probable that they are formed 

 from the cytoplasmic particles of smaller size (the microsomes). Hultin 

 (1953^, b) has summarised a number of studies in wliich radioactive iso- 

 topes have been used to follow the processes of protein synthesis in the 

 cell as a whole and in the various groups of particles (see also Kavanau 

 1953). During the early cleavage stages, it is in the microsomes that this 



Figure 5.6 



Graphs showing the relative density of the mitochondrial population 



(R.M.D., plotted vertically) at various levels of the animal-vegetative axis 



at 7, 14 and 26 hours of development (blastula to mid-gastrula). (From 



Gustafson 1954, after Lenicque, Horstadius and Gustafson 1953.) 



synthesis is proceeding most rapidly, but by the early gastrula the mito- 

 chondria are becoming very active. Hultin suggests that the mitochondria 

 are built up either from, or at least by the influence of, the microsomes. 

 The reactions between these two types of particles are, however, probably 

 reciprocal, since there is reason to believe that, once the mitochondria 

 have been formed, they induce a high rate of activity in the microsomes, 

 and that it is actually at these small particles that protein synthesis proceeds 

 most rapidly. The evidence from work with isotopes suggests that this 

 synthesis begins to get fully under way at about the time of gastrulation. 



