1()4 KAUI, (;. LARK 



1)11(1 iSU'iii, 19(12 1 or may inilicatc a I'olc of dcoxyi'ibosidi's otlitT than 

 as a pn'cui'sor I'oi' DXA (SttTii, 19(iOa). If the former is true, a similar 

 difforciu'e shoiikl ai)pear in the syntliesis of DNA which should then 

 occur as a two-step process. Tliis has not been observed. Moreover, the 

 amount of DNA synthesized is less than the deoxyriboside pool or the 

 amount of DNA broken down to create the i)Ool (tiiis loss of DNA luay 

 indicate the total size of the deoxyriboside pool formed) (Stern, 1960a). 



These difficulties appear to be explained by the recent work of 

 Takats (1962) who has demonstrated that the deoxyriboside material 

 which is derived from the breakdown of tapetal DNA is not used in the 

 synthesis of microspore DNA. He labeled the tapetum with thymidine- 

 H-' at a time when no DNA was being synthesized in the microspores and 

 subsequently traced the breakdown and reutilization of this radioactive 

 material by means of autoradiography. It was found that the degraded 

 material was taken up in the microspore walls but not into microspore 

 DNA. He suggests that this material could sen'e to induce the formation 

 of a second deoxyriboside or -tide pool (this might explain the double 

 peak of acid-soluble material prior to DNA synthesis) which is then 

 used to supply the necessary precursors for microspore DNA synthesis. 

 This idea is similar to that of Hotta and Stern (1961), who have sug- 

 gested that this pool of material is responsible for the induction of 

 nucleotide phosphoiylating enzymes in the anther. 



It would appear, then, that control of DNA synthesis in the lily 

 anther may be the result of a precursor pool induced by the products 

 from the degradation of tapetal DNA. The induction of a second pool 

 of deoxyriboside material is further suggested by the finding, by Stem 

 and Hotta (1962), of two types of DNA in cells of the anther wall, one 

 type having the base ratios and properties of the microspore DNA. 

 Breakdown of this material could easily provide a pool of precursors 

 for the synthesis of microspore DNA. 



A situation similar to that found in the division of lily microspores 

 may exist during the development of fertilized eggs and embryos of 

 amphibia and Echinodermata. Previous evidence (Hof!"-J0rgensen and 

 Zeuthen, 1952; Gregg and L0trup, 1955; Finamore and Volkin, 1958) 

 had indicated a high extra nuclear (cytoplasmic) content of DNA-like 

 substances in developing amphibian embiyos. It was presumed that this 

 material ser\'es as a source of nuclear DNA. It has been shown (Grant, 

 1958; Kuriki and Okazaki, 1959) that a large portion of this "excess 

 DNA" is composed of deoxynucleotides which are present in amounts 

 far in excess of the nuclear DNA. The presence of all of the pyrimidine 

 and purine deoxynucleotide triphosphates was noted. Thus this material 

 could serve as either a reservoir of pi-ccursors for DNA synthesis or as an 



