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tween 1828 and 1845, during which period 1,373,091 three-ruble pieces were minted, besides a few six-ruble and 12-ruble pieces. The three-ruble piece was worth $2.33 and it weighed 10.3 grams, for platinum was then worth but $7 an ounce; with platinum at $165 an ounce, the intrinsic value of such a coin to-day would be more than $54 of our money.

In view of the fact that the platinum output continues to be much smaller than some years ago, while the increasing demand for jewelry purposes offsets the falling off in the demand for munitions processes, it appears likely that the price will continue to go up, at least until the full resumption of platinum mining in Russia serves as a check. The search for the discovery of new sources is being diligently prosecuted, and Colombia seems the most hopeful of all the regions except Russia.

The newspaper notoriety given to platinum, because of the great legitimate demand for it and the consequent astonishing rise in value, before long excited the cupidity of dishonest persons. As a consequence of this there have

been numerous thefts of the material. In several cases, valuable specimens of platinum have been purloined from museum collections, and chemical utensils made of platinum have been stolen from a number of chemical laboratories. Indeed, in one instance an entire university laboratory was burned down to hide the theft of platinum.

As to future prospects, an extensive development of the platinum resources in the Republic of Colombia is in active progress. Possibly Canada may contribute somewhat by improved methods of refining the coppernickel ores, and similar ores mined else where may also furnish considerable platinum. However, the most encouraging sign is the reported determination of Soviet Russia to issue platinum certificates, that is to say, certificates secured by the platinum stock that has been accumulated in Russia and has not fallen into the hands of the Allies, or will be mined now and in the future.

GEORGE F. KUNZ

SEXUALITY IN MUCORS. II

66 NEUTRAL" RACES

As regards the intensity of sexual reaction, however, a gradation is clearly shown. A more detailed view of the complete table showing the combinations only where reactions might be expected, can be seen more clearly (Table I.). The higher grades of A and B predominate at the upper left-hand corner while at the opposite corner are only O's with C's and D's between. There is therefore in this species, varying degrees of sexual activity from the strongest down through the weakest to so-called "neutrals" which fail to show any sexual reaction under the conditions of the experiment. The word "neutral" is obviously only a relative term since, if the two races Nos. 811 and 367 had not been used as testers, No. 370 would have been classed as a neutral. It is possible that the 3 so-called neutrals would have taken part in zygospore formation if strong enough testers of the proper sex had been available or if more favorable environmental factors had been present. The fewer the number of tests made

and the more unfavorable the environmental conditions, the larger will be the number of races listed as neutral from any collection of races of a given species.

A change in sexual activity tending toward neutrality may be brought about by environmental factors. Thus we have obtained a temporarily neutral condition in both the plus and minus races of Mucor Mucedo by growing them for several non-sexual generations at unfavorably high temperatures. The sexual activity can be regained in a few generations by cultivating them at low temperatures. In the same species the spores in a germ sporangium frequently are neutral in reaction but later become sexually active. One of my most active forms (Mucor V) has become much reduced in sexual activity since its opposite races were first separated some sixteen years ago. A similar reduction in sexual vigor resulting in neutrality has been reported in a number of species by other investigators. In Phycomyces the plus and minus spores in a germ sporan

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gium may produce mycelia, neutral toward each other, though reacting with the plus and minus parent stocks. Here again the neutrality or self-sterility is only temporary, since after a few spore generations their ability to take part in active zygospore formation inter se is completely established.

In nature the number of neutral strains appears to be large and many species have been studied, the races of which have never been induced to react, either inter se or with strong testers of other species. There is no evidence that even in these cases, the neutrality is absolute and the races completely devoid of sexual tendencies. Their apparent neutrality may mean merely that we have not yet happened to expose these forms to the peculiar environmental conditions necessary

for an expression of the sex which is actually present.

SEXUAL DIMORPHISM

Similar tabulations of the sexual reactions between their races have been made for several species beside the "dark" Absidia and with similar results (Table II). All the races which were able to assist in zygospore forma tion appear to be sexually dimorphic, consistently either plus or minus. A few investigators have believed they have found evidence in certain species that would militate against sexual dimorphism in the diecious mucors. In a specific instance (8), however, where it was possible to retest the material which upon such conclusions were based, it was found that the dimorphism was in fact present but, for

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various reasons, has been misinterpreted by the investigator.

Burger, in a recent paper (11) concludes that sexual dimorphism does not exist in the mucor genus Cunninghamella. He reports finding certain races, among 25 or 26 of C. bertholettia studied, which will form zygospores with both plus and minus races. In other words a race A will conjugate with race B, B conjugates with C and C conjugates with A, and the family triangle is complete. In personal conversation, Dr. Burger has told me that he has found a similar condition in Syncephalastrum. It is not appropriate at the present time to enter into a discussion of Burger's paper. It will be sufficient to say that we have used some of the same strains that he worked with and, except for infections in an early series of contrasts before we dis

covered the great danger in Cunninghamella of contamination of a culture with spores of the opposite sex, we have never had results at all comparable with his. The negative results obtained by us do not, of course, prove that sex intergrades or hermaphrodites never occur in diecious species. He would be a rash philosopher who would deny to any protoplasm the possibility of reacting in an unexpected manner. They do indicate, we believe, that the occurrence of such sexual conditions must be, at best, a rare phenomenon. In view of the work tabulated in the accompanying table, it seems wisest therefore, to leave out of discussion, for the present, unconfirmed conflicting conclusions which are based on relatively meager material.

In the first 5 species of the table (those marked with a star) all the possible combina

tions have been made. For the others it would obviously have been too enormous a task to have been profitable. The races from zygospore germinations have been added as being likely to show through segregation sexual abnormalities if such existed. Nearly 10,000 combinations have been made using nearly 2,000 different races of diverse types of mucors and no race of a diecious species has been found which, if it showed any sexuality at all, reacted other than as a plus or a minus.

We have just been discussing intra-specific sexual reactions. The next table shows interspecific reactions previously discussed under the term "imperfect hybridization." In testing the reactions between the plus and minus races of two different species, all the four possible interspecific combinations have been made but, since the combinations between races with like signs have never given reactions, they have been omitted from the table. Only a part of the possible combinations have yet been tested, but sufficient to indicate that the same sexual dimorphism exists in all the species investigated.

We feel justified in concluding from our experience, that the forms in the tables are sexually dimorphic. From our experience with the diecious sporophytes of willows and poplars, such a strict dimorphism was hardly to have been expected. It would be a safe wager that one could not examine even a hundred individuals of either of these genera without finding sex intergrades. The apparent sharper differentiation of sex in the diecious mucors in comparison with higher plants is perhaps connected with the fact that in mucors we are dealing with sexually differentiated gametophytes instead of with sporophytes.

GAMETE DIFFERENTIATION

I should like to close our discussion by a consideration of gamete differentiation in mucors and other forms. As a general rule, all of the diecious mucors represented at the top of the chart (Fig. 4) have gametes equal in size. Of the hermaphrodites there are two types-those with equal gametes (isogamic),

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figure below, and those with a constant and marked difference in size (heterogamic), figures at right and left. We can conceive of the hermaphrodites as having been derived from the diecious types or the diecious types from the hermaphrodites. If the latter be the actual course of evolution, we may conceive a differentiation of sex to have taken place in two directions beginning with the isogamic hermaphrodites-first toward a differentiation, chiefly physiological, separating the sexes on separate plus and minus individuals in diecious forms; second toward a differentiation, conspicuously morphological, bringing about a constant difference in the size of the gametes in the heterogamic hermaphrodites. The prevalent biological distinction between males and females is based ultimately upon the relative size of the gametes which they produce. The smaller gamete is considered the male; the larger recognized in the left figure by the outgrowths behind it, is considered the female. The diagram (6) shows the reactions obtained in attempting, by use of this criterion of sex, to homologize the plus and minus signs with the terms male and female or vice versa. The hermaphrodite, which is heterogamic, is grown between the plus and minus races of Mucor V. On the right its smaller gamete reacts with the plus

race and on the left its larger gamete reacts, though but weakly, with the minus race. The smaller gamete is therefore minus and the larger plus. On the assumption that the smaller gamete is male and the larger female, the minus race must be considered male and the plus race female.

In our previous diagram (Figure 4) we reoognize on the left the heterogamic species (Absidia spinosa) just discussed, by the outgrowths back of the larger gamete. That heterogamy has actually been derived from isogamy in this species is rendered probable by Lendner's report (12) of finding a race of the same species with equal gametes. The broken lines, connecting the unequal gametes on the left with the plus and minus diecious species above, represent the reactions which have taken place and indicate that the larger gamete is plus and the smaller gamete, minus. The isogamic hermaphroditic species below also reacts with the diecious form above and hence its gametes also may be labelled plus and minus. The plus gamete of the lower isogamic species may be considered, in the process of evolution, to have given rise to the larger gamete of the left-hand figure as indicated by the solid line. This is an orthodox interpretation and consistent with the facts so far discovered for this species. There are some facts, however, which indicate that such is not the necessary course of evolutionary development in all forms.

It has been shown that although the plus race is perhaps usually more vigorous than the minus, this condition is sometimes reversed. Some hermaphrodites have predominatingly plus and some predominatingly minus tendencies. Is there any intrinsic reason why, of two equal gametes, the plus should invariably become the larger in the process of size differentiation? I do not be lieve that there is. If not, we should expect to find forms like the one figured on the right where the plus gamete is represented as having given rise to the smaller of the heterogamic pair. In Zygorhynchus heterogamus, we have perhaps such an example. The evidence is not entirely conclusive since we have

obtained reactions as yet only with one of the paired test races and the larger suspensor fails to show outgrowths which might help in distinguishing the unequal gametes when reacting with other forms. However, the appearance of the reactions between the righthand figure and the minus race resembles that between the left-hand figure and the plus race. The figure on the left has a minus tendency, the same as its smaller gamete while the figure on the right has a plus tendency also the same apparently as its smaller gamete. No one realizes more strongly than the speaker that the specific case under discussion is in need of more thorough investigation. Whether or not my suggested interpretation of the right-hand figure proves to be the correct one, it will serve to call attention to the fact that those who define male and female in terms of size differentiation in the sex cells are making the gratuitous assumption that quantitative differences in the gametes are the fundamental peculiarities of the two sexes. I have used from preference, therefore, the terms plus and minus because I have wished to speak in terms of the physiological differentiation into sexually dimorphic races established in diecious species rather than in terms of male and female which are defined by differentiation in size of gametes and which conceivably may be secondary sex characters.

I trust it will be granted that there is something fundamental, common to all the plus races that causes them to react sexually with minus races in the same or in different species and that this same fundamental something is present also in hermaphroditic forms whether possessed of equal or of unequal gametes. Dr. Gortner and I some years ago started an investigation based upon the assumption that the fundamental differences between the sexes might possibly be bound up with differences in sex proteins. The work was unfortunately interrupted before a definite conclusion could be reached with the delicate blood reactions employed. If we are able to imagine some fundamental biochemical constitution such as a sex protein, common to all the plus proto

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