THE SYNTHESIS OF NEW SUBSTANCES 369 



while only one or two appear in most somatic cells. The nucleoli contain 

 basic proteins and RNA, but little or no DNA. 



The last constituent of the nucleus is the sap. Unfortunately little is 

 known about this, although it must be very important since it presumably 

 contains both the substrates out of which new chromosomes are built 

 and also the immediate products of gene activity. The main constituents 

 are undoubtedly proteins (cf. Brown, Callan and Leaf 1950) and there is 

 no nucleic acid. 



The whole nucleus is enclosed by a nuclear membrance, which seems, 

 in several, and perhaps in all, cases to be a double structure. An outer lipo- 

 protein layer, a few hundred A thick, has a porous structure, the pores 

 having a diameter also of three or four hundred A; this is supported on a 

 thiimer {c. 150 A) layer which shows no obvious structure in the electron 

 microscope and is probably composed of an elastin-like protein (Callan 

 1951), on the nuclei of amphibian oocytes; Bairati and Lehmann (1952) 

 beheves that in Amoeba the porous layer lies inside the structureless one). 

 There is some evidence that the nuclear membrane is freely permeable to 

 proteins, which makes it easier to see how the genes can effectively control 

 the functioning of the cytoplasm (Anderson 1953, Stern and Mirsky 



1953)- 



The cytoplasm consists of a clear 'ground substance', in which granules 

 of various kinds are suspended. With microscopes using visible light, it 

 is difficult to obtain much further information about the former. Studies 

 with the electron microscope in recent years have been revealing a variety 

 of laminar or fibrillar structures, often taking the form of very thin 

 double membranes (Fig. 17.2). It is not yet clear, perhaps, to what extent 

 these structures are the results of the types of fixation used to prepare the 

 material (mostly neutralised osmic fixatives, which reveal almost no 

 structure in the nucleus); but it seems certain that there must be quite 

 elaborate structures of some kind or other in the apparently clear cyto- 

 plasm. (Sjostrand and Hanzon 1954.) 



Most recent discussions have emphasised the importance of the various 

 kinds of particles suspended in the cytoplasm. The first category of these 

 are globules of fat or lipo-protein yolk, wliich act as reserves of energy- 

 rich raw materials. A more active role is played by the mitochondria, 

 bodies which are large enough to be easily visible in the microscope 

 (c. 05-3 [J. in diameter, and sometimes ten or more times as much in 

 length). They contain protein, lipids, and a little RNA. They are usually 

 thought to contain little DNA, but Chayen and Norris (1953) have re- 

 cently shown that in actively metabohsing interphase cells, much of this 

 substance is located in cytoplasmic granules, from which it easily passes 



