Heredity and Sex (1913) 8
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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Chapter VIII Special Cases of Sex-Inheritance
The mechanism of sex-determination that we have examined gives equal numbers of males and females. But there are known certain special cases where equality does not hold. I have selected six such cases for discussion. Each of these illustrates how the mechanism of sex-determination has changed to give a different result ; or how, the mechanism remaining the same, some outside condition has come in that affects the sex ratio.
It is so important at the outset to clearly recognize the distinction between sex-determination and sex ratio, that I shall take this opportunity to try to make clear the meaning of this distinction. The failure to recognize the distinction has been an unfailing source of misunderstanding in the literature of sex.
(1) A hive of bees consists of a queen, thousands of workers, and at certain seasons a few hundred drones or males. The workers are potentially females, and these with the queen give an enormous preponderance of females. In this case the explanation of the sex ratio is clear. Most of the eggs laid by the queen are fertihzed, and in the bee all fertilized eggs become females, because as we have seen there is only one class of spermatozoa produced, and not two as in other insects.
There is a parallel and interesting case in one of the wasps described by Fabre. The female lays her eggs as a rule in the hollow stems of plants, each egg in a separate compartment. In some of the compartments she stores away much more food than in others. From these compartments large females hatch. From compartments where less food is stored the smaller males are produced. It may seem that the amount of food stored up determines the sex of the bee. To test this Fabre took out the excess of food from the large compartments. The wasp that emerged, although small for want of food, was in every case a female. Fabre enlarged the smaller compartments and added food. The wasp that came out was a male, larger than the normal male.
It is evident that food does not determine the sex, but the mother wasp must fertilize the eggs that she lays in chambers where she has stored up more food, and not fertilize those eggs that she deposits in compartments where she has accumulated less food.
(2) A curious sex ratio appeared in one race of fruit flies. Some of the females persisted in producing twice as many females as males. This was first discovered by Miss Rawls. In order to study what was taking place, I bred one of these females that had red eyes to a white-eyed male of another stock. All the offspring had red eyes, as was to be expected. I then bred these daughters individually to white-eyed males again (Fig. 106). Half of the daughters gave a normal ratio ; the other half gave the following ratio :
Fig. 106. — Diagram to show the heredity of the lethal factor (carried by black X). A, red-eyed female, carrying the factor in one X, is bred to normal white-eyed male. B, her red-eyed daughter, is bred again to a normal white-eyed male, giving theoretically the four classes shown in C, but one of the classes fails to appear, viz. the red-eyed male (colored black in the diagram). The analysis (to right) shows that this male has the fatal X. One of his sisters has it also, but is saved by the other X. She is the red-eyed female. If she is bred to a white-eyed male, she gives the results shown in D, in which one class of males is again absent, viz. the red-eyed male. In this diagram the black X represents red eyes and lethal (as though completely linked).
It is evident that one class of males has failed to appear — the red males. If we trace their history through these two generations, we find that the single sex chromosome that each red male contains is one of the two chromosomes present in the original red-eyed grandmother. If this chromosome contains a factor which if present causes the death of the male that contains it, and this factor is closely linked to the red factor, the results are explained. All the females escape the fatality, because all females contain two sex chromosomes. If a female should contain the fatal factor, her life is saved by the other, normal, sex chromosome. The hypothesis has been tested in numerous ways and has been verified. We keep this stock going by mating the red females to white males. This gives continually the 2 : 1 ratio. The white sisters, on the other hand, are normal and give normal sex ratios.
(3) Another aberrant result, discovered by Mr. Bridges, is shown by a different race of these same fruit flies. It will be recalled that when an ordinary whiteeyed female is bred to a red-eyed male all the sons have white eyes. But in the race in question a different result follows, as shown by the diagram. From 90 to 95 per cent of the offspring are regular, but 5 per cent of the females and 5 per cent of the males are unconformable, yet persistently appear in this stock.
The results can be explained if we suppose that the two sex chromosomes in the egg sometimes stick together (Fig. 107). They will then either pass out into one of the polar bodies, in whii^h case the red-eyed males will develop if the egg is fertilized by a female-producing sperm; or the two sex chromosomes will both stay in the egg, and give a kind of female with three sex chromosomes.
Here also numerous tests can be made. They verify the expectation. Thus by utiUzing sex chromosomes that carry other sex-Hnked characters than white eyes, it can be shown that the results are really due to the whole sex chromosome being involved, and not to parts of it. The result is of unusual interest in another direction ; for it shows that the female-producing
Fig. 107. — Non-disjunction of the sex chromosomes. In consequence a female produces three instead of one class of eggs (see to right of diagram) with respect to X. The results of the fertilization of such a female by a normal red male are shown in the lower part of the diagram.
sperm will make a male if it enters an egg from which both sex chromosomes have been removed. It is therefore not the female-producing sperm, as such, that gives a female under normal conditions, but this sperm plus the sex chromosome already present in the egg that gives an additive result — a female.
(4) In the group of nematode worms belonging especially to the genus Rhabditis, there are some extraordinary perversions of the sex ratios. The table gives the ratios that Maupas discovered. Not only are the males extremely rare — almost reaching a vanishing point in certain cases — but they have lost the instinct to fertilize the female.
to 1000 females
Diplogaster robustus 0.13 male
Rhabditis guignardi 0. 15 male
Rhabditis dulichura 0.7 male
Rhabditis caussaneli 1.4 males
Rhabditis elyaus 1.5 males
Rhabditis coronata 5.0 males
Rhabditis perrieri 7.0 males
Rhabditis marionii 7.6 males
Rhabditis duthiersi 20.0 males
Rhabditis viguieri 45.0 males
The females, on the other hand, have acquired the power of producing sperm, so that they have passed over into the hermaphroditic state. The behavior and history of the sperm that the females produce has only recently been made out by Miss Eva Kriiger. It is found that a spermatozoon enters each egg and starts the development, but takes no further part in the development (Fig. 108). The egg may be said to be half fertihzed. It is a parthenogenetic egg and produces a female.
(5) Some very high male ratios have been reported by Guyer in cases where birds of very different families have been crossed — the conimon fowl by the guinea hen, individuals of different genera of pheasants bred to each other and to fowls, etc. Hybrids between different genera gave 74 ^ — 13 9 . Hybrids between different species of the same genus 12$ — 18 9. In most of these cases, as Guyer points out, the sex is recorded from the mounted museum specimen which has the male plumage. But it is known that the reproductive organs of hybrids, extreme as these, are generally imperfect and the birds are sterile. It has been shown that if the ovary of the female bird is removed or deficient, she assumes the plumage of the male. Possibly, therefore, some of these cases may fall under this heading, but it is improbable that they can all be explained in this way. In the cases examined by Guyer himself the hybrids were dissected and all four were found to be males.
Fig. 108. — Oogenesis and spermatogenesis of Rhabditis aberrans. 1-5, stages in oogenesis, including incomplete attempt to form one polar body. Eighteen chromosomes in 1 and again in 4 and 5. In 3 the entering sperm seen at right. 6, prophase of first spermatocyte with 8 double and two single chromosomes (sex chromosomes). At the first division (7) the double chromosomes separate, and the two sex chromosomes divide, giving ten chromosomes to each daughter cell (8). At the next division the two sex chromosomes move to opposite poles, giving two female-producing sperm (9 and 10). Rarely one of them may be left at the division plane and lost, so that a male-producing sperm results that accounts for the rare occurrence of males. (After E. Krixger.)
Pearl has recently pointed out that the sex ratio in the Argentine RepubUc varies somewhat according to whether individuals of the same race, or of different races, are the parents. As seen in the following table, the sex ratio of Italian by Italian is 100.77;
Comparison of the Sex Ratios of the Offspring of Pure and
Cross Matings
Sex Ratio
Difference
Matings
P.E. OF Difference
Italian $ Argentine 9 Italian $ Italian 9
105.72 ±.46 100.77 ±.20
Difference
4.95 ±.50
9.9
Itahan $ Argentine 9 Argentine $ Argentine 9
105.72 ±.46 103.26 ±.34
Difference
2.46 ±.57
4.3
Spanish $ Argentine 9 Spanish ^ Spanish 9
106.69 ±.74 105.55 ±.36
Difference
1.14 ±.82
1.4
Spanish J Argentine 9 Argentine $ Argentine 9
106.69 ±.74 103.26 ±.34
Difference
3.43 ±.81
4.2
Argentine by Argentine, 103.26 ; but ItaUan by Argentine, 105.72. If, as has so often been found to be the case, a hybrid combination gives a more vigorous progeny, the higher sex ratio of the cross-breed may account for the observed differences, since other data show that the male infant is less viable and the increased vigor of a hybrid combination may increase the chance of survival of the male.
(6) We come now to the most perplexing case on record. In frogs the normal sex ratio is approximate equality. Professor Richard Hertwig has found that if the deposition of the eggs is prevented for two to three days (after the eggs have reached the uterus) the proportion of males is enormously increased — in the extreme case all the offspring may be males. By critical experiments Hertwig has shown that the results are not due to the age of the spermatozoa, although in general he is inclined to attribute certain differences in sex-determination to the sperm as well as to the eggs.
The evidence obtained by his pupil, Kuschakewitsch, goes clearly to show that the high male sex ratio is not due to a differential mortality of one sex.
In the following table four experiments (a, h, c, d) are summarized. The interval between each record
a) 47 9 : 32 ^ 9 : 97 ^
/^\ /^^\ /^^\
b) 34 9:47^ 65^:77^ 156 9:194^ 7 9:48^
c)64 9: 6U 101^:139^ 115 9:169^
/IK /'K /'N
d) 55 9:52^ 148 9:87^ 719:70^ 17 9:129^
is written above in hours. In all cases an excess of males is found if the eggs have been kept for several hours before fertilization. In the first (a), second (6), and fourth (d) cases the excess of males is very great. Hertwig attempts to bring his results into line with his general hypothesis of nucleo-plasm relation. He holds, for instance, that sex may be determined by the relation between the size of the nucleus and the protoplasm of the cell. As the value of the evidence has been seriously called into question in general, and as there is practically no evidence of any weight in its favor in the present case, I shall not dwell further on the question here. But the excessively high male ratio is evident and positive. How to explain it is difficult to say. It is just possible, I think, that delay may have injured the egg to such an extent that the sperm may start the development but fail to fuse with the egg nucleus. Under these circumstances there is the possibihty that all the frogs would be males.
Miss King has also carried out extensive sets of experiments with toads and frogs. She has studied the eggs and the sperm under many different conditions, such as presence of salt solutions, acids, sugar solutions, cold, and heat. Her results are important, but their interpretation is uncertain. In sugar solutions and in dry fertiUzation she has increased the proportion of males to 114 per 100 9 . The normal sex ratio for the toad is 90 ^ to 100 $ . Whether the solutions have in any sense affected the determination of sex, or acted to favor one class of sperm at the expense of the other remains to be shown, as Miss King herself frankly admits.
In the case of man there are extensive statistics concerning the birth rate. The accompanying tables give some of the results. There are in all parts of the world more males born than females. The excessively high ratios reported from the Balkans (not given here) may be explained on psychological grounds, as failure
to 100 females
]\Iales
Italy ........ 105.8
France 104.6
England 103.6
Germany 105.2
Austria 105.8
Hungary 105.0
Switzerland 104.5
Belgium 104.5
Holland 105.5
Spain 108.3
Russia 105.4 j
to report the birth of a boy is more likely to lead to the imposition of a fine on account of the conscription.
There can be no doubt, however, that slightly more males than females are born. Moreover, if the stillborn infants alone are recorded, surprisingly large ratios occur, as shown in the next table.
Males
Italy 131.1
France 142.2
Germany 128.3
Austria 132.1
Hungary 130.0
Switzerland 135.0
Belgium 132.0
Holland 127.1
Sweden . 135.0
Norway 124.6
Denmark 132.0
to 100 females
And if abortive births are also taken into account, the ratio is still higher. It seems that the male embryo is not so strong as the female, or else less likely, from other causes, to be born alive.
In many of the domesticated animals also, especially the mammals, there is an excess of males at birth, as the next table shows.
Males Females
Horse 98.31 100 (Busing)
Cattle 107.3 100 (Wilchens)
Sheep 97.7 100 (Irwin)
Pig 111.8 100 (Wilchens)
Rat 105.0 100 (Cuenot)
Dove 105.0 100 (Cuenot)
Hen 94.7 100 (Darwin)
A little later I shall bring forward the evidence that makes probable the view that in man the mechanism for sex-determination is like that in other animals, where two classes of sperm are produced, male- and female-producing. How then can we account in the human race for the excess of eggs that are fertilized by male-producing spermatozoa ? At present we do not know, but we can at least offer certain suggestions that seem plausible.
In mammals the fertilization occurs in the upper parts of the oviduct. In order to reach these parts the sperm by their own activity must traverse a distance relatively great for such small organisms. If the rate of travel is ever so slightly different for the two classes of sperm, a differential sex ratio will occur.
Again, if from any cause, such as disease or alcoholism, one class of sperm is more affected than the other, a disturbance in the sex ratio would be expected.
At present these are only conjectures, but I see no ground for seizing upon any disturbance of the ratio in order to formulate far-reaching conclusions in regard to sex-determination itself. As I pointed out in the beginning of this chapter, we may go wide of the mark if we attempt to draw conclusions concerning the determination of sex itself from deviations such as these in the sex ratio, yet it is the mistake that has been made over and over again. We must look to other methods to give us sufficient evidence as to sex-determination. Fortunately we are now in a position to point to this other evidence with some assurance. With the mechanism itself worked out, we are in a better position to explain slight variations or variables that modify the combinations in this way or in that. i
The Abandoned View that External Conditions Determine Sex
But before taking up the evidence for sex-determination in man I must briefly consider what I have been bold enough to call the abandoned view that external conditions determine sex.
Let us dismiss at once many of the guesses that have been made. Drelincourt recorded 262 such guesses, and Geddes and Thomson think that this number has since been doubled. Naturally we cannot consider them all, and must confine ourselves to a few that seem to have some basis in fact or experiment.
The supposed influence of food has been utilized in a large number of theories. The early casual evidence of Landois, of Mrs. Treat, and of Gentry has been entirely set aside by the careful observations of Riley, Kellogg and Bell, and Cuenot. In the latter cases the experiments were carried through two or even three generations, and no evidence of any influence of nourishment was found.
The influence of food in sex-determination in man has often been exploited. It is an ever recurrring episode in the ephemeral Hterature of every period. The most noted case is that of Schenk. In his first book he said starvation produced more females ; in his second book he changed his view and supposed that starvation produces more males.
Perhaps the most fertile source from which this view springs is found in some of the earlier statistical works, especially that of Diising. Dtising tried to show that more girls are born in the better-fed classes of the community, in the poorer classes more boys. The effective difference between these two classes is supposedly one of food ! For instance, he states that the birth-rate for the Swedish nobility is 98 boys to 100 girls, while in the Swedish clergy the birth-rate is 108.6 boys to 100 girls.
Other statistics give exactly opposite results. Punnet t found for London (1901) more girls born amongst the poor than the rich. So many elements enter into these data that it is doubtful if they have much value even in pointing out causes that affect the sex ratio, and it is quite certain that they throw no light on the causes that determine sex.
In other mammals where a sex ratio not dissimilar to that in man exists, extensive experiments on feeding have absolutely failed to produce any influence on the ratio. We have, for instance, Cuenot's experiments with rats, and Schultze's experiments with mice. The conditions of feeding and starvation were much more extreme in some cases than is likely to occur ordinarily, yet the sex ratio remained the same.
Why in the face of this clear evidence do we find zoologists, physicians, and laymen alike perpetually discovering some new relation between food and sex? It is hard to say. Only recently an ItaUan zoologist, Russo, put forward the view that by feeding animals on lecithin more females were produced. He claimed that he could actually detect the two kinds of eggs in the ovary — the female- and the male-producing. It has been shown that his data were selected and not complete ; that repetition of his experiments gave no confirmative results, and probably that one of the two kinds of eggs that he distinguished were eggs about to degenerate and become absorbed.
But the food theories will go on for many years to come — as long as credulity lasts.
Temperature also has been appealed to as a sex factor in one sense or another. R. Hertwig concluded that a lower temperature at the time of fertilization gave more male frogs, but Miss King's observations failed to confirm this. There is the earher work of Maupas on hydatina and the more recent work of von Malsen on Dinophilus apatris. I have already pointed out that Maupas' results have not been confirmed by any of his successors. Even if they had been confirmed they would only have shown that temperature might have an effect in bringing parthenogenesis to an end and instituting sexual reproduction in its stead. In hydatina the sexual female and the male producing individual are one and the same. A more striking case could not be found to show that the environment does not determine sex but may at least change one method of reproduction into another.
There remain von Malsen' s results for dinophilus.
where large and small eggs are produced by the same female (Fig. 109). The female lays her eggs in clusters, from three to six eggs, as a rule, in each cluster. The large eggs produce females; the small eggs pro
FiG. 109. — Dinophilus gyrociliatus. Females (above and to left) and males (below and to right). Two kinds of eggs shown in middle of lower row. (After Shearer.)
duce rudimentary males that fertiUze the young females as soon as they hatch and before they have left the jelly capsule.
Von Malsen kept the mother at different temperatures, with the results shown in the table. The ratio of small eggs to large eggs changes. But the result
Temperature
No. OF
Broods
d
?
Sex Ratio
Eggs per Brood
Room temp. 19° C. . Cold, 13° C. . . . Heat, 26° C. ...
202 925 383
327 973 507
813 2975
886
1:2,4 1:3,5 1:1,7
5,6 4,2 3,6
obviously may only mean that more of the large eggs are likely to be laid at one temperature than at another. In fact, temperature seemed to act so promptly according to Von Malsen's observations that it is very unlikely that it could have had any influence in determining the kind of egg produced, but rather the kind of egg that was more likely to be laid. We may dismiss this case also, I believe, as not showing that sex is determined by temperature.
Sex-Determination in Man
Let us now proceed to examine the evidence that bears on the determination of sex in man. I shall draw on three sources of evidence :
1. Double embryos and identical twins.
2. Sex-linked inheritance in man.
3. Direct observations on the chromosomes.
The familiar case of the Siamese twins is an example of two individuals organically united. A large series of such dual forms is known to pathologists. There are hundreds of recorded cases. In all of these both individuals are of the same sex, i.e. both are males or both are females. There is good evidence to show that these double types have come from a single fertilized egg. They are united in various degrees (Fig. 110) ; only those that have a small connecting region are capable of living. These cases lead directly to the formation of separate individuals, the so-called identical twins.
Galton was one of the first, if not the first, to recognize that there are two kinds of twins — identical twins and ordinary or fraternal twins.
Identical twins are, as the name implies, extremely alike. They are always of the same sex. There is every presmnption and some collateral evidence to show that they come from one egg after fertilization. On the other hand, amongst ordinary twins a boy and a girl, or two boys and two girls, occur in the ratio expected, i.e. on the basis that their sex is not determined by a common external or internal cause. Since fraternal twins and identical twins show these relations at birth and 'from the fact that they have been in both cases subjected to the same conditions, it follows with great probability that sex in such cases is determined before or at the time of fertilization.
DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF OIPLOPAGI ANO DUPLICATE TWINS, ILLUSTRATIVE OF THE THEORY. ADVANCED IN THIS PAPER. FURTHER EXPLANATION IN THE TEXT.
Fig. 110. — Diagram showing different types of union of double monster (After Wilder.)
This conclusion finds strong support from the conditions that have been made out in the armadillo. Jehring first reported that all the young of a single litter are of the same sex (Fig. 111). The statement has been verified by Newman and by Patterson on a large scale. In addition they have found, first, that only one egg leaves the ovary at each gestative period ; and second, that from the egg four embryos are pro
FiG. 111. — Nine-banded Armadillo. Four identical twins with a common placenta. (After Newman and Patterson.)
duced (Fig. 112). The material out of which they develop separates from the rest of the embryonic tissue at a very early stage. The four embryos are identical quadruplets in the sense that they are more like each other than like the embryos of any other litter, or even more like each other than they are to their own mother.
The second source of evidence concerning sex-determination in man is found in the heredity of sex-linked characters.
The following cases may well serve to illustrate some of the better ascertained characters. The tables are taken from Davenport's book on " Heredity in Relation to Eugenics." The squares indicate males, affected males are black squares ; the heavy circles indicate females, that are supposed to carry the factors, but such females do not exhibit the character themselves. Solid black circles stand for affected females.
Fig. 112. — Nine banded Armadillo. Embryonic blastocyst that has four embryos on it, two of which are seen in figure. (After Newman and Patterson.)
Haemophilia appears in affected stocks almost exclusively in males (Fig. 113). Such males, mating with normal females, give only normal offspring, but the daughters of such unions if they marry normal males will transmit the disease to half of their sons. Affected females can arise only when a haBmophilious male marries a female carrying haemophilia. If we substitute white eyes for haemophilia, the scheme already given for white versus red eyes in flies applies to this case. If, for instance, the mother with normal eyes has two X chromosomes (Fig. 114), and the factor for haemophilia is carried by the single X in the male (black X of diagram), the daughter will have one affected X (and in consequence will transmit the factor), but also one normal X which gives normal vision. The sons will all be normal, since they get the X chromosomes from their mother. In the next generation, as shown in the diagram (third line), four classes arise, normal females, hybrid females, normal males, and hsemophilious males. Color blindness follows the same scheme, as the above diagrams illustrate (Figs. 114 and 115). In the first diagram the colorblind male is represented by a black eye ; the normal female by an eye without color. The offspring from two such individuals are normal, but the color blindness reappears in one-fourth of the grandchildren, and in these only in the males. The reverse mating is shown in the next diagram in which the female is color-blind. She will have color-blind sons and normal daughters (criss-cross inheritance), and all four kinds of grandchildren.
Fig. 114. — Diagram to indicate heredity of color blindness through male. A color-blind male (here black) transmits his defect to his grandsons only. ;
Fig. 115. — Diagram to indicate heredity of color blindness through female. A color-blind female transmits color blindness to all of her sons, to half of her granddaughters and to half of her grandsons.
Fig. 116. — Pedigree of Ichthyosis from Bramwell. (After Davenport.)
Other cases in man that are said to show sex-linked inheritance are atrophy of the optic nerve, multiple sclerosis, myopia, ichthyosis (Fig. 116), muscular atrophy (Fig. 117), and night-blindness (Fig. 118). There are also other cases in man that appear to come under the same category, but for which the evidence is not so clear.
Fig. lis. — Pedigree of night blindness in a negro family from Bordley. (After Davenport.)
All these cases of sex-linked -inheritance in man are explained by the assmnption that the factor that produces these characters is carried by the sex chromosome, which is duplex (XX) in the female and simplex (X) in the male. A simpler assumption has not yet been found. If one is fastidious and objects to the statement of factors being carried by chromosomes, he has only to say, that if the factors for the characters follow the known distribution of the sex chromosome, the results can be accounted for.
The culmination of the evidence of sex-determination in man is found in a study of the cell structure of the human race itself. Strange as it may seem, we have been longer in doubt concerning the number of chromosomes in man than in any other animal as extensively studied. Four conditions are responsible :
(1) The large number of chromosomes present in man.
(2) The clumping or sticking together of the chromosomes. (3) The difficulty of obtaining fresh material. (4) The possibility that the negro race has half as many chromosomes as the white race.
Two years ago Gu^^er announced the discovery that in all probabihty there exist in man two unpaired chromosomes in the male (Fig. 119) that behave in all respects like that in the typical cases of the sort in insects, where, as we have seen, there are two classes of spermatozoa, differing by the addition of one more chromosome in one class. These produce females ; the lacking class produces males. But Guyer's evidence was not convincing. He found in all 12 chromosomes in one class of sperm and 10 in the other. Montgomery has also studied the same problem, but his account, while confirming the^ number, is in disagreement in regard to the accessory.
Jordan has gone over a number of other mammals, in some of which he thinks that he has found indications at least of two classes of sperm.
Still more recently another investigator, von Winiwarter, has attacked the problem (Fig. 120). His material and his methods appear to have been superior to those of his predecessors. His results, while stated with caution and reserve, seem to put the whole question on a safer basis.
His main results are illustrated in the diagram (Fig. 120). In the male he finds 47 chromosomes. Of these 46 unite at reduction to give 23 double chromosomes — one remains without a mate. At the first reduction division the pairs separate, 23 going to each pole, the unpaired chromosome into one cell only.
Fig. 119. — Human spermatogenesis according to Guyer. The sex chromosomes are seen in 6-9.
At the next division all the chromos(3mes in the 23 group divide, Ukevvise all in the 24 group divide. There are produced two spermatozoa containing 24 chromosomes, and two containing 23 chromosomes; all four sperms having come from the same spermatogonia! cell (Fig. 121).
Fig. 120. — Human spermatogenesis according to von Winiwarter, a, spermatogonia! cell with duplex number; h, synapsis ; c, d, e,f, first spermatocytes with haploid number of chromosomes ; g, first spermatocyte division, sex chromosomes (below) in advance of others ; h, two polar plates of later stage ; i, first division completed ; j, second spermatocyte with 23 chromosomes ; k, second spermatocyte ^\^th 24 chromosomes ; I, second spermatocyte division ; m, two polar plates of later stage.
In the female von Winiwarter had difficulty in determining the number of chromosomes present. His
Fig. 121. — Diagram of human spermatogenesis. A, spermatogonial cell with 47 chromosomes; B, first spermatocyte with reduced haploid number and sex chromosome (open circle) ; C, first division ; D, two resulting cells = second spermatocytes ; E, division of second spermatocytes ; F, four resulting spermatozoa, two female-producing (above), two male-producing (below).
best counts gave 48 chromosomes for the full or duplex number. These observations fit in with the results from the male.
If these observations are confirmed, they show that in man, as in so many other animals, an internal mechanism exists by which sex is determined. It is futile then to search for environmental changes that might determine sex. At best the environment may sHghtly disturb the regular working out of the two possible combinations that give male or female. Such disturbances may affect the sex ratio but have nothing to do with sex-determination.
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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