J. R. SHAVER 279 



of gradients of intensity of dye reduction in animal and vegetal 

 halves of sea urchin embryos and concluded that the activity of 

 the fragments in this respect is not equivalent to that of the whole 

 egg. But it has been pointed out (Brachet, 1950) that the tech- 

 nique of anaerobic dye reduction is qualitative and that the in- 

 vestigators who have used it on sea urchin embryos are not 

 unanimous about the existence of such gradients. 



There is a further difficulty in reconciling reported metabolic 

 gradients with the mitochondrial giadients described by Gustaf- 

 son and Lenicque (1952). It will be recalled that, according to 

 these investigators, mitochondiial numbers are higher in the cells 

 of the animal hemisphere in mesenchyme blastulae and young 

 gastrulae. If it is supposed that enzymes are incorporated into 

 mitochondria at this time, it would seem reasonable to expect that 

 the areas in which they occur in higher numbers would be char- 

 acterized by higher metabolic activity. It would also follow, from 

 Gustafson's theory, that these areas would be the locale of highest 

 differentiation potential. But the reverse situation seems to exist, 

 according to the results of Child (1941) and Horstadius (1952). 

 These investigators found a vegetal-animal gradient of dye reduc- 

 tion at the beginning of gastrulation, which Child thought was 

 independent of respiratory activity. Thus, a gradient, opposing 

 the animal-vegetal gradient, whose highest point coincides with 

 the point where primary diflFerentiation is ordinarily understood 

 to begin in the sea urchin embryo, apparently requires no mito- 

 chondria to function. 



Clearly, many points remain to be clarified before we can un- 

 derstand how the many complex interactions which must be 

 involved in the morphogenetic gradients of the sea urchin em- 

 bryo operate in differentiation. 



Quantitative Aspects of Mitochondrial 

 Populations in Development 



There is a faiily extensive older literature concerning the 

 duplication and distribution of cytoplasmic particles during de- 

 velopment and the corollary problem of the differentiation of 

 mitochondria into fibrillar structures, secretory granules, etc. 



