378 PRINCIPLES OF EMBRYOLOGY 



taining the nucleus of a difFerent species {A. Wettsteinii). When the hat 

 was now removed, the important point emerged that the new one which 

 regenerated had the characters of Wettsteinii, that is to say of the nucleus 

 lying at the base of the alga and not of the stalk to which the regenerate 

 was attached. It is clear that the nucleus (presumably the genes contained 

 in it) has caused the production of some substance which controls the 

 morphogenesis of the regenerate. Unfortunately nothing is known of 

 the chemical nature of the substances in question. 



It has recently been found, e.g. by Mirsky (1951), that difFerent tissues 

 characterised by their richness in particular substances may contain these 

 substances not only in the cytoplasm but also in the nuclei. For instance, 

 haemoglobin is found in the nucleus in the early stages of the develop- 

 ment of red blood corpuscles. We have already mentioned such facts 

 as examples of the differential activation of genes by their associated 

 cytoplasm; the point which is being made here is that it also suggests 

 that the substances concerned are manufactured in close proximity to the 

 genes themselves and may be immediate rather than secondary gene 

 products. 



4. Genes and enzymes 



The substances which have been identified in nuclei in this way are 

 mostly enzymes. There is a good deal of other evidence which indicates that 

 genes may frequently operate by means of their effect on cellular enzymes, 

 although as we shall see, these are more probably formed in the cytoplasm 

 rather than directly by the genes. One of the earliest cases in which the 

 effect of a gene could be described in biochemical terms was that of 

 alkaptanuria in man; the homozygote for a certain recessive gene is unable 

 to oxidise homogentisic acid to allantoin, and the former is therefore 

 excreted unchanged in the urine (Garrod 1923). It seems that the normal 

 allele of the alkaptanuric gene is essential for the production of the en2r^me 

 which brings about the oxidation. Several other similar cases have been 

 described in mammals, and in recent years very many have been dis- 

 covered in lower organisms such as moulds, fungi, yeasts, bacteria, etc. 

 (Reviews: Catcheside 1951, Beadle 1949, Horowitz 1950, Haldane 1954). 

 Normal strains of these organisms can grow on relatively simple media, 

 from which they can synthesise all the substances necessary for their con- 

 tinued existence. If a strain, say of the fungus Neurospora, is x-rayed or 

 otherwise caused to mutate, and the haploid spores produced are tested 

 for their nutritional requirements, many mutants will be found which 

 cannot grow unless certain specific substances are added to the basic 

 medium, It is concluded that some step in the chain of reactions, which in 



