Heredity and Sex (1913) 7

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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.

Heredity and Sex (1913): 1 Evolution of Sex | 2 Mechanism of Sex-Determination | 3 Mendelian Principles of Heredity and Bearing on Sex | 4 Secondary Sexual Characters Relation to Darwin's Theory of Sexual Selection | 5 Effects of Castration, Transplantation on Secondary Sexual Characters | 6 Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex | 7 Fertility | 8 Special Cases of Sex-Inheritance | Bibliography
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter VII Fertility

Darwin's splendid work on cross- and self-fertilization, his study of the mechanism of cross-fertilization in orchids, and his work on the different forms of flowers of plants of the same species, mark the beginning of the modern study of the problem of fertility and sterility. Darwin carried out studies on the effects of cross-fertilization in comparison with self-fertilization and reached the conclusion that the offspring resulting from cross-fertilization are more vigorous than the offspring from self-fertilization. No one can read his books dealing with these questions without being impressed by the keenness of his analysis and the open-minded and candid spirit with which the problems were handled. Since Darwin's time we have not advanced very far beyond the stage to which Darwin carried these questions. We have more extensive experiments and some more definite ways of stating the results, but Darwin's work still stands as the most important contribution that has been made to this subject.

The credit of the second advance belongs to Weismann. His speculations concerning the effects of mixing of the germ-plasms of the two individuals, that combine at the time of fertilization, not only aroused renewed interest in the nature of the process of sexual reproduction, but brought to light also the effects of recombination of the different sorts of qualities contained in the parental strains. His attack on the hypothesis of rejuvenation that was so generally held at that time did very great service in exposing the mystical nature of such an imagined effect of crossfertilization. In particular, Weismann's endeavor to connect the theory of recombination with the facts of maturation of the egg and sperm has opened our eyes to possibilities that had never been realized before. His work has led directly to the third advance that has been made in very recent years, when the results of Mendelian segregation have been applied directly to the study of fertility and sterility.

As I have said, Darwin's work showed that crossfertilization is generally beneficial. The converse proposition has long been held that continued inbreeding leads to degeneration and to sterility. This opinion rests largely on the statements of breeders of domesticated animals and plants, but there is also a small amount of accurate data that seems to support this view. I propose first to examine this question, and then consider what cross-fertilization is supposed to do, in the light of the most recent work.

Weismann inbred white mice for 29 generations, and Ritzema-Bos bred rats for 30 generations. In each case the number of young per litter decreased in successive generations, more individuals were sterile and many individuals became weakened. This evidence falls in line with the general opinion of breeders.

On the other hand, we have Castle's evidence on inbreeding the fruit fly through 59 generations. He found some evidence of the occurrence of sterile pairs (mainly females), but we must be careful to distinguish between the appearance of sterile individuals in these cultures and the lessened fertility that may be shown by the stock in general. The recent work of Hyde on these same flies has shown that the appearance of sterile individuals may be an entirely different question from that of a decrease in general fertility. The latter again may be due to a number of quite different conditions. Castle and his co-workers found that the sterile individuals could be eliminated if in each generation the offspring were selected from pairs that had not produced sterile individuals. Hyde has found, in fact, that one kind at least of sterile females owe their sterility to a definitely inherited factor that can be eliminated as can any other Mendelian recessive trait. Moenkhaus, who has also extensively studied the problem of inbreeding in these flies has likewise found that his strains could be maintained at their normal rate of propagation by selecting from the more fertile pairs.

If we eliminate from the discussion the occurrence of sterile individuals, the question still remains whether the output of the fertile pairs decreases if inbreeding is carried on through successive generations. There is some substantial evidence to show that this really takes place, as the following figures taken from Hyde's results show.







— -^13








At the end of thirteen generations the fertility of the stock was reduced by half, as determined in this case by the average number of flies per pair that hatch. But this is not a measure of the number of eggs laid or of those that are fertiUzed.

Whether inbreeding where separate sexes exist is similar to self-fertihzation in hermaphroditic forms is not known. Darwin gives results of self-fertilization in Ijwmoea purpurea for ten generations. The effects vary so much in successive generations that it is not possible to state whether or not the plant has become less fertile. His evidence shows, however, that the crossfertilized plants in each of the same ten generations are more vigorous than the self-fertilized plants, but this does not prove that the latter deteriorated.

The problem has been studied in other ways. Some animals and plants propagate extensively by parthenogenesis ; others by means of simple division.

Whitney and A. F. Shull kept parthenogenetic strains of Hydatina senta for many generations. Whitney carried a strain of this sort through 500 generations. Towards the end the individuals became weak, the reproductive power was greatly diminished, and finally the strain died out. No attempt was made to breed from the more fertile individuals, although to some extent this probably occurred at times. If we admit that weakened individuals appear sometimes in these lines and their weakness is inherited, then each time such an individual happened. to be picked out a step downw^ard would be taken ; when the more fertile individuals chanced to be selected, the strain w^ould be temporarily held at that level. But on the w^hole the process would be downw^ards if such downward changes are more likely to occur than upward ones.

This is an assumption, but perhaps not an unreasonable one. Let me illustrate why I think it is not unreasonable. If the highest possible point of productivity is a complex condition due to a large number of things that must be present, then any change is more likely to be downward, since at the beginning the high-water mark had been reached. In time casual selectioij would be likely to pick out a poor combination — if this happened once the likelihood of return would be small.

As we have seen (Chapter I) Maupas found in a number of protozoa that if he picked out an individual (after each two divisions) to become the progenitor of the next generation, the rate of division after a time slowed down. The individuals became weaker and finally the race died out. Calkins repeated the experiments with paramoecium on a larger scale and obtained similar results. The question arose whether the results were not due to the hay infusion lacking certain chemical substances that in time produced an injurious effect. Calkins tested this by transferring his weakened strains to different culture media. The result was that the race was restored to more than its original vigor. But very soon degeneration again set in. A new medium again restored vigor to some degree, but only for a short time, and finally the oldest culture died out in the 742d generation. It was evident, therefore, that if the slackened rate of division and other evidences of degeneration were in part due to the medium, yet some of the effects produced were permanent and could not be effaced by a return to a more normal medium. Then came Woodruff's experiments. He kept his paramoecia on a mixed diet — on the kind of materials that it would be likely to meet with in nature, alternating with hay and other infusions. He found no degeneration, and at his last report his still vigorous strain was in the 3000th generation.

How can we harmonize these different results ? It is hazardous, perhaps, to offer even suggestions, but if we assume that in a medium not properly balanced paramcecium is likely to degenerate in the sense that it loses some of its hereditary factors, we can understand the failure to become normal when this has once taken place even in a new environment. Temporarily the decrepit individual may be benefited by a change, but not permanently if its hereditary mechanism is affected. In Woodruff's experiment the normal environment brings about no degenerative changes in the hereditary mechanism and the race continues to propagate indefinitely.

Let us turn now to the other side of the question and see what results cross-fertilization has given.

Hyde has found that if two strains of flies with low fertility are crossed, there is a sudden increase in the output, as seen in the diagram (Fig. 96). The facts show clearly an improvement. More eggs of each strain are fertilized by sperm from the other strain than when the eggs are fertilized by sperm from the same strain.^ In this case the results are not due to a more fertile individual being produced (although this may be true) but to foreign sperm, acting better than the strain's own sperm. The evidence, as such, does not show whether this is due to each strain having degenerated in certain directions, or to some other kind of a change in the heredity complex.

The egg counts show that in the inbred stock many of the eggs are not fertiUzed, or if fertiUzed (32%) they still fail to develop. This means a decrease in fertility in the sense in which that word is here used. The offspring that arise from the cross-fertilization of these strains are more vigorous than their parents, if their increased fertility be taken as the measure of their vigor. The latter result is not shown in the table, for here 52% and 58% are the percentages of fertile eggs produced when the two strains are crossed. i

41 i story of lrit>red Stock.

Fig. 96. — The horizontal line Fi-Fu gives the average number of flies per pair that emerged from inbred stock, decreasing from 368 to 156 per pair. Below is shown the results of a cross between a race of Truncates (short wings) and F13. The percentages here give the number of eggs that hatched in each case.

Darwin found that cross-fertilization was beneficial in 57 species of plants that he studied. In the case of primula, which is dimorphic, he found not only that self-fertiHzation gave less vigorous plants, but that when pollen from a long-styled flower of one plant fertilizes the pistil of another long-styled plant the vigor of the offspring is less than when the same kind of pollen is used to fertilize the pistil of a short-styled flower. The next table gives the detailed results.

^ The upper line Fi-Fiz gives the average output of flies per pair. Below this line the percentages mean the number of isolated eggs that hatched.

Nature of Union

Number of

Flowers Fertilized


OF Seed Capsules


OF Seeds in

Any One



OF Seeds IN Any One Capsule


No. OF

Seeds per


Long-styled form by pollen of shortstyled form : Legitimate union.






Long-styled form by own-form pollen : Illegitimate union.






Short-styled form bjpollen of longstyled form : Legitimate union.






Short-styled form by own-form pollen : Illegitimate union.






The two legitimate unions together.







The two illegitimate unions together.






We know now that these two types of plants — longstyled and short-styled — differ from each other by a single MendeUan factor. We may therefore state Darwin's result in more general terms. The heterozygous plant is more vigorous than the homozygous plant. Moreover, in this case it is not the presence of the dominant factors that makes greater vigor (for the short-styled plant containing both dominants is less vigorous than the heterozygous), but the presence of two different factors that gives the result.

Fig. 97. — At left of figures there are two strains of pure bred corn and at I'ight the hybrids produced by crossing those two pure strains. (After East.)

The most thoroughly worked out case of the effects of inbreeding and cross-breeding is that of Indian corn. In recent years East and G. H. Shull have studied on a very large scale and with extreme care the problem in this plant. Their results are entirely in accord on all essential points, and agree with those of Collins, who has also worked with corn.

East and Shull find that when two strains of corn (that have been to a large extent made pure) are crossed, the offspring is more vigorous than either parent (Fig. 97). This is clearly shown in the accompanying pictures. Not only is the hybrid plant taller and stronger, but in consequence of this, no doubt, the yield of corn per bushel is much increased, as shown in the next figure (Fig. 98).

Fig. 98. — At left an ear of Leamiug Dent corn, and another at right after four years of inbreeding. The hybrid between the two is shown in the middle ear. (After East.)

When the vigorous Fi corn is self-fertihzed, it produces a very mixed progeny, more variable than itself. Some of the F2 offspring are like the original grandparental strains, some like the corn of first generation, and others are intermediate (Fig. 99).

Fig. 99. — No. 9 and No. 12, two inbred strains of Learning Dent corn compared with Fi and F2 (to right). (After East.)

It will not be possible for us to go into an analysis of this case, but ShuU and East have shown that the results are in full harmony with Mendelian principles of segregation. The vigor of the T^i corn is explained on the basis that it is a hybrid product. To the extent to which the two parent strains differ from each other, so much the greater will be the vigor of the offspring.

This seems an extraordinary conclusion, yet when tested it bears the analysis extremely well.

ShuU and apparently East also incline to adopt the view that hybridity or heterozygosity itself is the basis for the observed vigor ; but they admit that another interpretation is also possible. For instance, each of the original strains may have been deficient in some of the factors that go to make vigor. Together they give a more vigorous individual than themselves.

Whitney ran one line of hydatina through 384 parthenogenetic generations, when it died (Line A). Another line was carried through 503 generations, and at the last report was in a very weakened condition (Line B). When the former line was becoming extinct, he tried inbreeding. From the fertilized eggs he obtained a new parthenogenetic female. It showed scarcely any improvement. The other line gave similar results. In one case he again inbred for a second time. He found that the rates of reproduction of lines A and B were scarcely, if at all, improved.

Whitney then crossed lines A and B. At once an improvement was observed. The rate of reproduction (vigor) was as great as that in a control line (reared under the same conditions) that had not deteriorated.

The experiments of A. F. ShuU on hydatina were somewhat different. He began with the twelfth generation from a sexual egg. The line was supposedly not in a weakened condition. He inbred the line and obtained from the fertilized egg a new parthenogenetic series. After a few generations he inbred again. The results are shown in the next table. It is clear that there has been a steady decline despite sexual reproduction, measured by four of the five standards that Shull applied, namely, size of family of parthenogenetic females, and of sexual females, number of eggs per day,

Showing Decrease of Vigor, as Measured by Various Characters, IN Six Successively Inbred Parthenogenetic Lines OF Hydatina senta



Character to be Measured

Size of family of parthenogenetic female . . Size of family of fertilized sexual female . .

Number of eggs laid per day

Number of days required to rear-h maturity'

Proportion of cases in which first daughter

did not become parent

Same in percentages

Size of family of parthenogenetic female . . Size of family of fertilized sexual female . .

Number of eggs laid per day

Number of days required to reach maturity Proportion of cases in which first daughter

did not become parent I 1/11

Same in percentages

Number of Pa















































































number of times the first daughter was too weak to become the mother of a new Une. It is clear that inbreeding did not lead to an increase in vigor.

In paramoecium there is also some new evidence. Calkins in 1904 brought about the conjugation of two individuals of a weak race in the 354th generation. From one of the conjugants a new line was obtained that went through another cycle of at least 376 generations in culture, while during the same time and under similar conditions the weakened race from which the conjugants were derived underwent only 277 generations.

Jennings has recently reported an experiment in which some paramoecia, intentionally weakened by breeding in a small amount of culture fluid, were allowed to conjugate. Most of the lines that descended from several pairs showed no improvement but soon died out. In only one case was an individual produced that was benefited by the process.

Jennings' results are, however, peculiar in one very important respect. He did not use a race that had run down as a result of a long succession of generations, but a race that he had weakened by keeping under poor conditions. We do not know that the result in this case is the same as that in senile races or inbred races of other workers. It is not certain that the hereditary complex was affected in the way in which that complex is changed by inbreeding. He may have injured some other part of the mechanism.

Jennings interprets conjugation in paramcecium to mean that a recombination of the hereditary factors takes place. Some of these combinations may be more favorable for a given environment than are others. Since these will produce more offspring, they will soon become the predominant race.

The next diagram (Fig. 100) will serve to recall the principal facts in regard to conjugation in paramcecium. Two individuals are represented by black and white circles. At the time of conjugation the small or micronucleus in each divides (B), each then divides again (C). Four nuclei are produced. One of these micronuclei, the one that lies nearest the fusion point, divides once more, and one of the halves passes into the other individual and fuses there with another nucleus. The process is mutual. Separation of the two individuals then takes place and two ex-con jugants are formed. Each has a new double nucleus. This nucleus divides (G) and each daughter nucleus divides again (H), so that each ex-con jugant has four nuclei.

Fig. 100. — Diagram to show the history of the micronuclei of two Paramoecia during (A-F) and after (F-J) conjugation. Compare this diagram with Fig. 2.

Another division gives eight nuclei in each. The paramoecium itself next divides — each half gets four nuclei. A second division takes place, and each gets two of the nuclei. Four new individuals result. In each of these individuals one of the nuclei remains small and becomes the new micronucleus, the other enlarges to form the new macronucleus. Thus from each excon jugant four new paramcecia are produced, which now proceed to divide in the ordinary way, i.e. the micronucleus and the macronucleus elongate and divide at each division of the animal.

It is customary to regard some phase in this process as involving a reduction division in the sense that a separation of the paired factors takes place. If this occurs prior to interchange of micronuclei (E), then each ex-con jugant corresponds to an egg after fertilization. It is conceivable, however, that segregation might occur in the two divisions that follow conjugation, which would give a different interpretation of the process than the one followed here.

On the first of these two hypotheses two new strains result after conjugation. Each is a recombination of factors contained in the two parents. If the two parents were alike, i.e. homozygous, in many factors, and different, i.e. heterozygous, in a few, the two individuals would be more alike than were the original races from which they came. This is, in fact, what Jennings has shown to be the case, at least he has shown that on the average the ex-con jugants are more like each other than were the original strains.

Calkins has obtained some .new and important facts concerning the likeness and unlikeness of the new strains that result from conjugation. He has used wild, i.e. not weakened, individuals, and has followed the history of the four lines resulting from the first four individuals produced by each ex-conjugant. The history of six such ex-con jugants is shown in the next diagram (Fig. 101). The four Unes, quadrants," (1, 2, 3, 4) that are descended from each of six exconjugants (viz. G, H, L, M, Q, B) are shown. At intervals large numbers of the populations were put under conditions favorable to conjugation and the number of conjugating pairs counted. The results are shown in the diagram. The circles indicate no conjugations ; X indicates the death of the strain. In the G and in the M series many conjugations took place. In other series conjugation did not take place until much later. Striking differences appear in the different quadrants although they were kept under similar conditions.

Fig. 101. — History of six (G, H, L, M, Q, B) ex-conjugants. In each the descendants of the first four individuals (after conjugation) is shown; the numbers indicate the pairs of conjugants counted when the test was made. X indicates deaths; indicates that no conjugation took place. (After Calkins.)

But even amongst the four lines descended from the same ex-con jugant marked differences exist. These differences cannot be attributed to constitutional differences unless a segregation of factors takes place after conjugation or unless it can be shown that these differences are not significant. In the light of these conflicting results on paramcecium it may seem unsafe to draw any far-reaching conclusions concerning the nature of sexual reproduction in general from the evidence derived from these forms. In the higher animals, however, the evidence that segregation takes place prior to fertilization and that recombinations result can scarcely be doubted.

Theories of Fertility

Let us now try to sum up the evidence in regard to the influence of cross-fertilization. This can best be done by considering the three most important hypotheses that have been brought forward to explain how crossing gives greater vigor.

Shull and East explain the vigor of the hybrid by the assumption that it contains a greater number of different factors in its make-up than either of its parents. They support the view by an appeal to the next {F2) generation from such hybrids that shows a lower range of vigor, because, while a few individuals of this generation will be as mixed as the hybrid (Fi), and therefore like it, most of them will be simpler in composition. This interpretation is also supported by the evidence that when pure lines (but not necessarily, however, homozygous lines) are obtained by self-fertilizing the offspring of successive generations from these first hybrids, further decUne does not take place.

An alternative view, that is also Mendelian, has been offered by Bruce and by Keeble and Pellew. Vigor, it is maintained, is in proportion to the number of dominant factors, and in proportion to the number of these factors present whether in a hybrid or in a homozygous (duplex) condition.

On this view the hybrid is vigorous, not because it is hybridous, so to speak, but because in its formation a larger number of dominant factors (than were present in either parent) have been brought together.

A third view is also compatible with the evidence, namely, that there may exist factors that are themselves directly concerned with fertility. There is one such case at least that has been thoroughly analyzed by Pearl.

Pearl studied for five years the problem of fertility in two races of fowls, viz. barred Plymouth rocks and Cornish Indian games. The main features of his results are shown in the diagram (Fig. 102). He finds that the winter output of eggs, which is correlated with the total production, is connected with two factors. One factor, designated by Li, is a non-sex-linked character. If it is present, an average of less than 30 eggs is produced in the winter season. There is another factor, L2, that is present in the barred rocks, but not in the Indian game. If present alone, the winter output is again about 30 eggs on an average. If, however, both Li and L2 are present, the winter output is more than 30 and may be as great as 90, or in rare cases 100-120 eggs.

The peculiarity about this discovery is that the second factor, L2, is sex-linked, which means in this case that it is carried by the eggs that will produce the males in the next generation, and not by the eggs that will produce the daughters. Hence if the daughters of highproducing hens are selected, one does not get in them the high productiveness of the mother. It is her sons that inherit the character, although they cannot show it except in their offspring.

Fig. 102. — Illustrating Pearl's hypothesis. F = female factor present in half of the eggs and determining sex. Li = factor for low egg production; U, its allelomorph for zero production of winter eggs. L2 = factor for high winter production; U, its allelomorph.

Aside from whatever practical interest these results may have, the facts are important in showing that such a thing as a factor for fertility itself may be present, without otherwise being apparent, and that this factor taken in connection with another (or others) gives high productivity.

The other point to which I wish to call attention relates to a different matter. We have met with some cases where lowered fertility was due to eggs failing to a greater or less degree to be fertilized by sperm of the same strain.

Fig. 103. — Normal male of Drosophila (on left) and male with "rudimentary" wings (on right). Note sex comb (lower left). .

A striking case of this kind is found in a mutant of the fruit fly that appeared in my cultures. The mutant has rudimentary wings (Fig. 103). The females are absolutely infertile with males of the same kind.

If they are mated to any other male of a different strain, they ane fertilized. The males, too, are capable of fertilizing the eggs of other strains, in fact, are quite fertile.

The factor that makes the rudimentary winged fly is of such a sort that it carries infertility along with it — ; in the sense of self-infertility. This result has nothing to do with inbreeding, and the stigma cannot be removed by crossing out and extracting.

A somewhat similar factor, though less marked, is found by Hyde in certain of his inbred stock to which I have referred. As his experiments show, the infertility in this case is not due to lack of eggs or sperm, but to a sort of incompatibility between them so that not more than 20 per cent of the eggs can be fertilized by males of the same strain.

In the flowering plants where the two sexes are often combined in the same individual, it has long been known that there are cases in which self-fertilization will not take place. The pollen of a flower of this kind if placed on the stigma of the same flower or of any other flower on the same plant will not fertilize the ovules. Yet the pollen will fertilize other plants and the ovules may be fertilized by foreign pollen.

Correns has recently studied that problem and has arrived at some important conclusions. He worked with a common plant, Cardamine pretensis. In this plant self-fertilization is ineffectual. He crossed plant B with plant G, and reared their offspring. He tested these with each other and also crossed each of them back to its parents that had been kept alive for this purpose. The latter experiment is simple and more instructive. His results and his theory can best be given together.

Correns assumes that each plant contains some factor that produces a secretion on the stigma of the flowers. This secretion inhibits the pollen of the same plant from extending its pollen tube. He found, in fact, that the pollen grains do not grow when placed on the stigma of the same plant. All plants will be hybrid for these factors, hence plant B will produce two kinds of germ-cells, B and h. Similarly, plant G will produce two kinds of germ-cells, G~g. If these two plants are crossed, four types will be produced. When these are back-crossed to the parents, the expectation is shown in the diagram (Fig. 104). Half the combination should be sterile and half should be fertile. This is, in fact, what occurs, as shown in the same diagram. The — signs indicate that fertilization does not occur, while the + signs indicate successful fertilization.

Fig. 104. - Illustrating the crossing of the types Bh and Gg to give four classes : BG, Bg, hG, hg. Each of these is then back-crossed either to B or to G with the positive ( + ) or negative (-) results indicated in the diagram.

Correns' theory is also in accord with other combinations that he made. There can be Httle doubt that he has pointed out the direction in which a solution is to be found.

There is a somewhat similar case in animals. In one of the Ascidians, Ciona intestinalis, an hermaphrodite, the sperm will not fertilize the eggs of the same individual. But the sperm will fertilize eggs of other individuals, and vice versa. Castle first found out this fact, and I have studied it on a large scale. The diagram (Fig. 105) gives an example of one such experiment made recently by W. S. Adkins.

Five individuals are here used. The eggs of one individual, A, were placed in five dishes (horizontal line) ; Hkewise those of B, C, D, E. The sperm of A, designated by a (vertical lines) was used to fertilize the eggs, A, B, C, D, E ; likewise the sperm b, c, d, e. The self-fertilized sets form the diagonal line in the diagram and show no fertilization. The other sets show various degrees of success, as indicated by the percentage figures. These results can best be understood, I think, by means of the following hypothesis. The failure to self-fertilize, which is the main problem, would seem to be due to the similarity in the hereditary factors carried by eggs and sperm ; but in the sperm, at least, reduction division has taken place prior to fertilization, and therefore unless each animal was homozygous (which from the nature of the case cannot be assumed possible) the failure to fertilize cannot be due to homozygosity. But both sperm and eggs have developed under the influence of the total or duplex number of hereditary factors ; hence they are alike, i.e. their protoplasmic substance has been under the same influences. In this sense, the case is hke that of stock that has long been inbred, and has

Fig. 105. — The oblique line of letters A", £^ C^ D'^ , E', gives the selffertilized sets of eggs; the rest A'^, A^, etc., the cross-fertilized sets. A, B, C, D, E = eggs ; a, b, c, d, e, = sperm of same individuals. (From unpublished work of W. S. Adkins.)

come to have nearly the same hereditary complex. If this similarity decreases the chances of combination between sperm and eggs, we can interpret the results. Correns' results may come under the same interpretation.

I have tried to bring together the modern evidence that bears on the problems of fertihty and sterility. It is evident that there are many obscure relations that need to be explained. I fear that, owing to the difficulty of summarizing this scattered and diverse material, I have failed to make evident how much labor and thought and patience has been expended in obtaining these results, meager though they may appear.

But while it is going to take a long time and many heads and hands to work out fully these problems, there can be little doubt that the modern method is the only one by which we can hope to reach a safe conclusion.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Heredity and Sex (1913): 1 Evolution of Sex | 2 Mechanism of Sex-Determination | 3 Mendelian Principles of Heredity and Bearing on Sex | 4 Secondary Sexual Characters Relation to Darwin's Theory of Sexual Selection | 5 Effects of Castration, Transplantation on Secondary Sexual Characters | 6 Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex | 7 Fertility | 8 Special Cases of Sex-Inheritance | Bibliography

Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.

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