J. R. SHAVER 277 



In addition to the conflicting cytological evidence concerning 

 the existence of mitochondrial gradients in sea urchin embryos, a 

 few other points may be noted which seem to present some ob- 

 jections to Gustafson's hypothesis. 



1. Although the initial phases of development (fertilization 

 and cleavage ) may go on in the absence of mitochondria and/or 

 microsomes ( Harvey, 1946 ) , it does not follow that these particles 

 aie not utilized in normal development. Alternate pathways may 

 exist in the cell which could accomplish the same ends. It is pos- 

 sible that soluble factors from mitochondria may remain in the 

 fragments after the removal of the granules (assuming that all 

 mitochondria are removed), which, in conjunction with the mi- 

 crosomes of Harvey (1946), present in the fine line of granules, 

 might substitute for the missing mitochondria for a limited pe- 

 riod. In any case, the cytological evidence that the mitochondria 

 in the embryo before differentiation are present mainly as pre- 

 cursors is conflicting. 



2. The use of the phrase "primary differentiation" can be mis- 

 leading if it is meant to imply that nothing has occurred in the 

 embryo up to gastrulation but the passive distribution of mate- 

 rials dm^ing cleavage. Sea urchin embryos, for example, are, in a 

 sense, free-living organisms for some time after hatching before 

 primary differentiation commences. The elaboration of cilia, for 

 locomotion, is a form of differentiation which presumably requires 

 at least some of the processes of transformation involved in any 

 specialization of cell type. If it is accepted that mitochondria per 

 se do not begin to function in differentiation until gastrulation 

 in the sea urchin, it must be admitted that other systems not re- 

 quiring mitochondria can also function in such cellular transfor- 

 mations. 



3. The elaboration of proteins, either structural or metabolic, 

 has been postulated by many cytologists to result directly from the 

 activity of cytoplasmic particles (see Wilson, 1928, for a review 

 of older literature; Holtfreter, 1948, for a recent application of 

 this idea to shape changes in embryonic cells), but it has been 

 pointed out that we as yet know little about the synthesis of pro- 

 teins in vivo (Fruton, 1954). Biochemical information indicates 



