1. The development and morphology of the semi-dominant mutations Ragged and Opossum are compared. The results of the present study support evidence of previous linkage studies which suggest that Ragged (Ra) and Opossum (Raop) are alleles.

2. Examination of embryos from 18-day-old first parity matings of + + × Raop + reveals that there is no deficiency of Raop + embryos. However, in Ra + × Raop + and Raop + × Raop + matings some Raop + embryos die in utero in addition to the pre-natal death of the RaRaop and RaopRaop embryos.

3. In all matings of Ragged and Opossum, there is a high frequency of oedematous Raop + embryos. In + + × Raop + matings, the frequency of oedema increases with increased age of the embryos.

4. Although the pattern of hair follicle formation is the same in both the mutant and normal embryos, follicles begin initiation later in the mutant embryos. The delay in follicle initiation is increased in the following order: Ra +, Raop + (non-oedemic), RaRa and Raop + (oedemic).

5. Heterozygous Opossum and Ra + mice have a full complement of pelage follicle primordia, but some follicles fail to differentiate. The differentiating follicles pass through the same developmental stages and require the same time for development as follicles in + + mice.

6. Heterozygous Ragged and Raop + mice have a full complement of tylotrichs, whereas the concentration of awls is variable. In Ra + mice there is a reduction in zigzags, and Raop + mice usually lack zigzags. No auchenes are observed in the mutant mice.

7. Hair growth is asynchronous in Ra + and Raop + mice, with the exception of the first pelage coat. The quiescent phase of the hair growth cycle is short or even missing.

8. The degree of Ra and Raop gene activity is in the following order of increased severity: Ra +, Raop + (semi-lethal), RaRa (semi-lethal), RaRaop (lethal) and RaopRaop (lethal).

The intensity of negative interference in 28 crosses involving 14 allelic paba mutants in different combinations has been measured. Among selected cross-overs between two such mutants, when the distance between the mutants is large, additional exchanges on the distal side (relative to the centromere) greatly exceed those on the proximal side. With decreasing length of the interval of selection, the inequality disappears. It is suggested that this polarity may be the outcome of a undirectional process which imposes a time sequence on the recombination events within an effectively paired segment. Some alternative explanations of this result are discussed.

(1) A tail-skin grafting method was used to test for histocompatibility differences between members of thirteen sublines of C3H inbred mice, kept in a 1 r./night gamma radiation field for twelve generations, on the average, and separated from each other by about thirty-four generations.

(2) No homograft rejections occurred, so there was no evidence to suggest that mutations at histocompatibility loci had taken place in any of these sublines. Calculations show that this finding does not conflict with the idea that a fairly large number of loci are involved, having mutation rates similar to those already known in the mouse.

A variable delay among mating pairs in the time between contact formation and the initiation of F transfer was found. The conjugation process has no effect on the multiplication of the recipient cell.

In liquid medium newly infecting F-particles multiply faster than the host cell.

The experiment strongly suggests that the number of episomes per cell is small and that the distribution of the F-factor among the daughter cells is non-random.

1. Mate-killer paramecia (derived from stock 540, P. aurelia) were treated with ribonuclease and 8-azaguanine to determine the effect of these two substances on the mu particles and metagons.

2. Ribonuclease destroyed the metagons in living paramecia but had no direct effect on the viability of the mu particles. The latter were however destroyed precisely after one fission of cells whose metagons had been eliminated by ribonuclease treatment.

3. Re-synthesis of metagons was found to take place between the second and third fissions after treatment with ribonuclease, if the dominant gene M2 was present.

4. Increasing the concentration of ribonuclease in the external medium, or in the duration of exposure of paramecia to ribonuclease, resulted in destruction of increasing proportions of metagons.

5. 8-azaguanine did not destroy metagons already present, but destroyed the mu particles immediately.

6. It is concluded that the results obtained with these two substances are consistent with the view that RNA is an essential constituent of metagons.

Ten A factor mutants and an unlinked A suppressor mutant were isolated from fruit bodies formed by common A heterokaryons of Coprinus lagopus. All A factor mutants formed false clamps and lacked A mating-type specificity. Four A5 and two A6 mutants had functional α sub-units which were recovered in tests for intra-factor recombination in matings between mutant and wild-type factors. No functional β sub-units were recovered in tests of two of the A5 mutants and the two A6 mutants which possessed functional α sub-units. From three other mutants (two A5 and one A6) neither α nor β sub-units were recovered.

The suppressor mutant also formed false clamps but was recessive since stocks carrying it retained their A and B specificities when mated with wild-type testers. Fruit bodies formed by monokaryons carrying the suppressor mutant had pale gills bearing varying proportions of abnormal tetrads with less than four spores.

An additive-multiplicative model is found to describe diallel cross data rather well. Estimates of the model's parameters are highly correlated from generation to generation.

1. The low but regular positive correlation between body-size and the duration of the larval period in populations of D. melanogaster has been studied by selecting for large size or shorter development time on aseptic defined diets deficient in RNA and comparing the results with parallel selection on unrestricted yeast diets or on media in which RNA is not a limiting factor.

2. There is a striking contrast according to the nature of the diet during selection. On unrestricted diets and where RNA is adequate there is little or no evidence of correlation between the two characters, but on low RNA media there is a striking correlation whether selection is for large body size or shorter development time.

3. This contrast is accounted for in terms of particular changes in larval growth which can be divided into a first stage of growth to a critical size in the early 3rd instar and a second stage thereafter. The duration of the first stage can be greatly prolonged by inadequate diet but the duration of the later stage appears to be virtually unaffected by such variation although the amount of growth and hence final body-size, may be drastically reduced. The different diets which lead to presence or absence of correlation have enabled selection either to extend the growing period, so that the critical stage is reached later at a larger larval size, or to accelerate the growth rate in the later stage.

4. Variation in the final stage of growth predominates on unrestricted diets and is responsible for the greater part of the variation in body size in unselected populations. Stabilization of body-size about an intermediate optimum refers especially to growth in this later stage.

5. Lines selected for fast development on low RNA media are especially sensitive to minor nutritional variation. Probably only under rather special conditions is it possible to shorten the duration of the larval period and this is compatible with the importance of development time in fitness generally.

6. There is evidence that the restriction of early growth, in the 2nd instar, reduces the size of the 3rd instar mouth-parts. Such reduction is correlated with changes in adult size probably because smaller mouth-parts restrict food intake.

7. The pattern of larval growth suggests a flexible system which can be adjusted to different ecological conditions since the same body-size can be attained by adjusting the amount of growth effected before or after the critical stage. Differ ences in this respect will involve characteristic differences in reaction to environ mental variation and particular nutritional conditions are likely to influence the way in which adaptive changes are realized.

The inactive-X theory of dosage compensation postulates that in all somatic cells of adult female mammals one or other of the two X chromosomes is genetically inactive. This means that in a female heterozygous for two non-allelic genes acting through the same cells, and carried one on each X chromosome, one or other gene should act in all cells. Conversely, if the two genes are carried on the same X, then both genes should act in some cells and neither gene in the remainder. This point has been tested by breeding experiments with mice, using pairs of genes affecting coat colour and coat texture. In female mice carrying the colour mutant dappled, Modp, on one X and a translocation including the wild-type alleles of pink-eye, p, and albino, c, on the other, either Modp or the translocation acted in all cells. With the genes tabby, Ta and striated, Str, affecting coat texture, in Str + / + Ta females tabby acted only in the non-Str patches, while in StrTa/ + + it acted only in the Str ones. Thus these experiments confirm that only one of the two X chromosomes is active in the somatic cells of female mammals.

1. The paper described an attempt to see whether differences in co-adaptation between populations of Drosophila subobscura are related to the distance between them. The mean and the variance of body-size, development time and survival were recorded on parent populations and the F1 and F2 of various crosses to test for heterosis in the F1 and decline in performance or greater variance in the F2, which might indicate the break-up of co-adapted gene arrays. Comparisons were carried out at different temperatures and on a variety of larval diets, especially sub-optimal ones in which the larvae were grown on synthetic media. A large number of wild flies were caught at sites separated by about 10 miles along a transect of southern Scotland; these comprised one series of comparisons. For more distant crosses flies were caught at sites in southern England, Denmark, Switzerland and Israel.

2. There were well-defined differences in body-size, and, to a lesser degree, development time between populations from more widely separated localities and these showed evidence of a cline, northern populations having larger body-size. The difference in size between the Scottish and Isreal populations is about 20%.

3. There was no evidence of differences in co-adaptation between populations even in crosses between populations from sites as far apart as Scotland and Israel. The F1's were always close to the mid-parent values and there was no evidence of breakdown in the F2 nor of increased variability.

4. There was hardly any evidence of gene-environment interaction either with respect to different diets or to different temperatures.

5. Records of body-size on flies caught in the wild showed that they are extremely variable, indicating great variation in larval nutrition. Under natural condition stability of growth in body-size is conspicuously lacking in this species.

6. An additional test of co-adaptation was based on the between-family variance of abdominal bristle number of intra- and inter-population matings in the two most widely separated populations. There was no evidence of greater variance in the inter-population series.

7. To test for possible differences in breeding structure, the response to inbreeding was determined for two widely separated populations of D. subobscura and a long-established cage population of D. melanogaster, on an unrestricted larval diet and also on several different kinds of sub-optimal diets. There was little or no sign of consistent differences between the species in their response to inbreeding.

8. This test revealed differences between the two species in their minimum requirements for particular nutrients. subobscura is less able than melanogaster to withstand lower levels of protein and survival is particularly reduced. On the other hand, melanogaster has a considerably higher requirement for choline. Where there are apparent differences between the species in the average effect of inbreeding, the inbreeding effect is greater on the relatively more sub-optimal diet.

9. Comparison of the performance of the immediate descendants of wild flies with those derived from the same site, but kept in the laboratory for some twenty generations, failed to show any differences on several different diets and so there was no evidence that adaptation to laboratory conditions was important.

10. The lack of evidence for co-adaptation apparently conflicts with what has been claimed for other species. Such differences are discussed.

1. The four mating-types I, II, III, IV known for P. bursaria, syngen 1 are determined by specific combinations of dominant and recessive alleles at two unlinked loci. Mating-type I is formed by cells with the genotypes AABB, AABb, AaBB and AaBb; type II is controlled by the genotypes aaBB and aaBb; the double recessive, aabb is type III, genotypes AAbb and Aabb bring about type IV.

2. Clones which are exceptions to these rules for mating-type determination occur in low frequency (2–3%) in one line of descent. All can be explained by assuming either a mutation of dominant gene B to its recessive allele b or to a loss of the B locus. The data suggest that in four clones, the macronuclear but not the micronuclear genotype is affected; in two clones both nuclei are aberrant.

3. The cytogenetic events of conjugation were verified by the use of the new genetic markers. No evidence for uniparental nuclear reorganization, autogamy, was found.

The detached cilia from sexually reactive cells of Paramecium bursaria will agglutinate with the cilia of intact sexually reactive animals of a complementary mating type. No such reaction will occur if incompetent cells are used or if the cells are of the same mating type. Particulates other than cilia do not adhere to tester cells; the cilia which carry the specific mating-type substances are restricted to the ventro-lateral surface of the animal.

Studies of the heat inactivation of the ability of detached cilia to agglutinate confirm in detail the hypothesis of Metz which holds that cell unions leading to conjugation are brought about by the interaction of two pairs of complementary substances, A and a and B and b; the former pair of substances is more heat labile than the latter. The data suggest that animals of each of the four mating types carry a unique combination of two substances, namely, AB, aB, ab, and Ab.