GROWTH, DEVELOPMENT, AND DIFFERENTIATION 199 



than swimming. At 64 hr, when the controls are beginning to gastrulate, 

 the treated embryos are still spherical blastulae, and at 72 hr have not 

 progressed beyond this point. In this species, malonate would appear to 

 be a rather specific inhibitor of gastrulation without altering cleavage pri- 

 marily. It may be noted that other cycle inhibitors, such as parapyruvate 

 and fluoroacetate, also specifically block gastrulation in Strongylocentrotus. 

 It is difficult to understand these marked differences between the behaviors 

 of various sea urchin eggs, and especially the striking effects obtained by 

 Rulon with such low malonate concentrations. The free malonate concen- 

 trations in his work would have been around 0.17 milf (he used Ca++-free 

 sea water) and the intracellular concentration presumably much less. 



There is evidence that insect spermatogenesis is an aerobic process with 

 the terminal electron transport through the cytochrome system, and that 

 the cycle is the primary pathway for substrate oxidations. Yet no inhi- 

 bition of meiosis or differentiation into spermatids and spermatozoa by 

 50 ToM malonate is observed in hanging-drop cultures from the Cecropia 

 silkworm (Schneiderman et at., 1953). These experiments were carried out 

 at pH 6.8-7.2 and it is possible that malonate failed to penetrate. 



Amphibian gastrulation is inhibited by high concentrations of malonate. 

 Frog embryos at the early dorsal lip stage were dissected to give explants 

 which were exposed to 40 raM malonate at pH 8.0 for 18 hr. Development 

 does not proceed beyond the next stage (Ornstein and Gregg, 1952; Gregg 

 and Ornstein, 1953). There is no differential effect on the respiration of 

 dorsal and ventral explants, both being inhibited about 60%. Unfortunately, 

 there is some doubt that the block of development is due to any specific 

 effect of the malonate since 45 vciM NaCl apparently inhibits to the same 

 degree. Thus the mechanism of the block could have been osmotic or a Na"*" 

 effect. The chick embryo seems to be much more sensitive to malonate. 

 Explants of chick embryo in the presence of glucose undergo morphogenesis 

 and differentiation to the formation of the central nervous system and an 

 actively beating heart. Malonate at 1-2 mJf exerts striking differential ef- 

 fects (Spratt, 1950). Although no differences were noted during the first 

 20 hr, afterwards the central nervous system degenerates completely while 

 the heart forms normally and beats. Malonate is the most specific inhibitor 

 for the development of the nervous system. Some antagonism of this effect 

 was seen with succinate but none with fumarate. No inhibition of mitoses 

 in cultures of chick bone is observed with malonate from 65 to 138 nxM 

 (A. F. W. Hughes, 1950). 



Mitoses in Mammalian Tissues 



Epidermal mitotic activity in mouse ear fragments was determined over 

 4 hr periods at pH 7.4 and 38^ by BuUough and Johnson (1951). From the 

 effects of anaerobiosis and various substrates it was concluded that the 



