278 EARLY ECHINODERM DEVELOPMENT 



( Siekevitz, 1952 ) that protein synthesis in vitro by microsomes is 

 facihtated by the presence of mitochondria or by a sokible factor 

 from them. It does not so far seem to have been suggested, on the 

 biochemical level, that mitochondria directly produce proteins. 

 Gustafson (1955) has observed bundles of fibers in sea urchin 

 eggs which have differentiated without cleavage, but an associa- 

 tion of fibrous structures with mitochondria in normal embryos 

 has not been reported. 



4. Although there is evidence that enzymes may increase or 

 decrease in activity during development and that the differentia- 

 tion of specific organs and tissues may be accompanied by the 

 appearance (or activation) of a particular enzyme or enzymes, 

 there is no evidence so far that "the development of an enzyme 

 is in any way causally linked with the differentiation process with 

 which it appears to be associated" (Boell, 1955). Gustafson and 

 Hasselberg (1951) showed that several enzymes increase in ac- 

 tivity at the mesenchyme blastula stage in the sea urchin, while 

 others maintain a constant level. It was not directly shown that 

 these enzymes are structurally associated with mitochondria in 

 the sea urchin, and the assumption that they are so bound has 

 been based on a comparison of curves for enzyme activities with 

 the curves of relative mitochondrial densities observed by Gus- 

 tafson and Lenicque ( 1952 ) . But the assumption that there is an 

 accumulation of enzymes in mitochondria at a particular period 

 of development does not seem to justify placing a causal inter- 

 pretation on the event. 



5. Gradient theories of differentiation have suffered generally 

 from the criticism that it is difficult to decide whether the pat- 

 terns of metabolic activity or cellular structures are the cause or 

 the, result of the determination of a region ( Brachet, 1950; Holt- 

 freter and Hamburger, 1955). Gustafson's hypothesis does not 

 seem to avoid this difficulty, and presents some additional prob- 

 lems in connection with physiological gradients in the sea urchin 



egg- 

 No difference in the respiration of animal and vegetal halves 

 of sea urchin eggs or embryos has been detected (Lindahl and 

 Holter, 1940 ) . Horstadius ( 1953, 1955 ) reexamined the question 



