Heredity and Sex (1913) 6
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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Chapter VI Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex
Three different sex conditions occur in animals and plants that have a direct bearing on problems of Heredity and Sex.
The first condition is called Gynandromorphism — a condition in which one part of the body is like the male, and the other part like the female.
The second condition is called Hermaphroditism — a condition in which the individuals of a species are all alike — maleness and femaleness are combined in the same body. Two sets of reproductive organs are present.
The third condition is called Parthenogenesis — a condition in which the eggs of an animal or plant develop without being fertilized.
Gynandromorphism
Gynandromorphs occur most frequently, in fact almost exclusively, in insects, where more than one thousand such individuals have been recorded. They are most abundant in butterflies, common in bees (Fig. 81) and ants, rarer in other groups. They occur relatively more often, when two varieties, or species, are crossed, and this fact in itself is significant. A few examples will bring the cases before us.
In my cultures of fruit flies several gynandromorphs have arisen, of which two examples are shown in Fig. 82. In the first case the fly is female on one side, as shown by the bands of her abdomen, and male on the other side (upper right-hand drawing).
In the second case the fly looked like a female seen from above. But beneath, at the posterior end, the genital organs of the male are present, and normal
Fig. 81. A gynandromorph niutillid wasp, Pseudomethoca canadensis, male on right side, female on left side.
in structure. In the latter case the fly is ostensibly a female, except for the male organs of reproduction.
How can we interpret these cases ? We find a clue, I think, in the bee. It is known that if the egg of the bee is fertilized, it produces a female — only female-producing sperms are formed. If it is unfertilized, it produces a male. In the bee two polar bodies are produced, and after their extrusion the number of chromosomes is reduced to half, as in ordinary cases. The haploid number produces a male ; the double number produces a female.
Boveri pointed out that if through any chance the entering sperm should fail to reach the egg nucleus before it divides, it may then fuse with one of the halves of the egg nucleus after that divides. From the half of the egg containing the double nuclei female structures will develop ; from the other half, containmg the half number of chromosomes, male structures (Fig. 83, .4). Here we have a very simple explanation of the gynandromorphism.
Fig. - gynandromorphs of Drosophila ampelophila. Upper ett-hand figure, female dorsally, male ventrally (as seen in third figure, lower fine). Upper right-hand figure, male on left side, female on right, and correspondmgly the under side shows the same difference (lower row, last figure to right. Lower row from left to right; normal female, normal male, vertical gynandromorph and lateral gynandromorph.
Fig. 83. — Diagram, illustrating on left (A) Boveri's hypothesis, on right (B) the author's hypothesis, of gynandromorphism.
There is another way in which we may imagine that the results are brought about. It is known that two or more spermatozoa frequently enter the egg of the bee. Should only one of them unite with the egg nucleus, the parts that descend from this union will be female. If any of the outlying sperm should also develop,
they may be supposed to produce male structures (Fig. S3,B).
The first case of the fly, in which one half the body is male and the other female, would seem better in accord with Boveri's hypothesis. In its support also may be urged the fact that Boveri and Herbst have shown that the belated sperm-nucleus may unite with one of the two nuclei that result from the first division of the egg nucleus.
On the other hand, the second case of the fly (where only a small part of the body is male) may be better accounted for by my hypothesis. It is known that single sperms that enter an egg without a nucleus, or even with one, may divide. The two hypotheses are not mutually exclusive, but rather supplementary.
Toyama has described a gynandromorph in the silkworm that arose in a cross between a race with a banded caterpillar (the female parent) and a race with a white caterpillar (the male parent). As shown in Fig. 84, the gynandromorph was striped on the left (maternal) side and white on the other (right) side. When the adult moth emerged, the left side was male and right side was female. Since the sperm alone bore the white character, which is a recessive character, it appears that the right side must have come from sperm alone. This is in accordance with my hypothesis.
In this connection, I should like to call attention to a relation of especial interest. Gynandromorphs are not uncommon in insects, rare or never present in birds and mammals.
The explanation of this difference is found, I think, in
Fig. 84. — I, a, plain, h, striped caterpillar of silkworm. II, a, gynandromorph silkworm, h, moth of same. Ill, wings of last. IV, dorsal view of same moth. V, abdomen of same. VI, end of abdomen of same moth. VII, normal female, and VIII, a normal male. (After Toyama.)
the relation of the secondary sexual characters to the sex glands. In insects the characters in question are not dependent on the presence or absence of these glands. Hence, when such conditions occur after fertihzation, as those I have just considered, each part may develop independently of the rest.
Hermaphroditism
In almost all of the great groups of animals a condition is found in which complete sets of ovaries and testes occur in the same individual. This condition is called hermaphroditism." In some groups of animals, as in flatworms, leeches, mollusks, hermaphroditism is the rule, and it is the common condition in flowering plants. Sometimes there is only one system of outlets for eggs and sperm, but not infrequently each has a separate system.
Here there is no problem of the production of males and females, for one kind of individual alone exists. But what determines that in one part of the body male organs develop, and in another part a female system ?
Two views suggest themselves, either somatic segregation, or regional differentiation. By somatic segregation I mean that at some time in the development of the embryo — at some critical division — a separation of chromosomes takes place so that an egg-producing group and a sperm-producing group is formed. There is no direct evidence in support of this view.
Another view is that the formation of ovary and testis is brought about in the same way as all differentiations of body organs, as for example the
168
HEREDITY AND SEX
formation of liver and lungs and pancreas from the digestive tract. The following case may perhaps be considered as supporting such an hypothesis. In a hermaphroditic worm, Criodrilus lacuum the ovaries lie in the thirteenth and the testes in the tenth and eleventh segments. If the anterior end be cut off, a new one regenerates, as shown by Janda (Fig. 85),
Fig. 1,
Fig. 2.
-Kg. 3.
Fig. 4.
Pig. 6
Fig. 85. — 1, anterior end of normal criodrilus, showing reproductive system; 2-5, regenerated anterior ends. (After Janda.)
in which the ovaries and testes reappear approximately in their appropriate regions. It is true their location is more liable to vary than in the normal worm, but this is unimportant. The important point is that they must be produced from parts of the body that have never produced them before, and it is unlikely therefore that any preparation for this casualty would have been made. The location and differentiation
HERIVIAPHRODITISM 169
of these organs may seem to depend on the same relation-of-the-parts-to-each-other " on which all somatic differentiation depends.
If this were the correct interpretation then the problem of sex in hermaphrodites would appear in a different light from the problem of sex in species in which males and females occur, and the appeal would be made to an entirely different principle.
In cases where a sexual generation alternates with a hermaphroditic generation, the problem of the two
Fig. 8Q. ~ Rhabditis nigrivenosa, male (left) and female (right). (After
Leunis.)
sexes reappears. There is but one case in animals that has been adequately worked out. A nematode worm, Rhabditis nigrovenosa, lives as a parasite in the lungs of frogs. It is an hermaphrodite. Its eggs give rise to another generation that lives in mud and slime. In this generation two kinds of individuals are present — true males and females (Fig. 86) . The females produce eggs, that are fertilized, and develop
170
HEREDITY AND SEX
into the hermaphrodites which find their way again into the lungs of frogs.
Boveri and Schleip have worked out the history of the chromosomes in this case. The cells of the
Fig. 87. — Chromosomes of Angiostomum. (.4), oogonia, (B), equatorial plate of first maturation division; (C), young spermatocyte; (D), first spermatocyte division in metaphase ; (E), same in anaphase; (F), spermatocyte of second division; (G), and (H), division of same; (/), and (K), loss of X at plane of division ; (L), first segmentation division of a male embryo ; two sets of chromosomes (5 and 6=11 respectively) separate ; (M) equatorial plate of dividing cell of female embryo = 12 chromosomes; (A), same from male embryo =11 chromosomes. (After Schleip.)
hermaphrodite have twelve chromosomes (Fig. 87). The eggs, after extruding two polar bodies, have six chromosomes. The spermatozoa that develop in the body of the same animal have six or five chromosomes each, because one chromosome is lost in half
HERMAPHRODITISJM 171
of the cells by being left at the dividing line between the two cells. We can understand how two kinds of individuals are produced by the hermaphrodites from the two classes of sperm combining at random with the eggs.
These two kinds of individuals are females with twelve chromosomes, and males with eleven chromosomes. How then can we get back to the hermaphroditic generation? Boveri and Schleip suggest that the males again produce two kinds of spermatozoa, — they have shown this to be the case in fact, — and that the male-producing spermatozoa become functionless. Here we have at least an outline of some of the events in the life cycle of this worm in relation to the chromosomes, but no explanation of hermaphroditism.
Turning to plants, there are the interesting experiments of the Marchals with mosses. They show that a hermaphroditic or sporophyte plant has the factors for maleness and femaleness combined as a result of fertilization ; while in the formation of the spores the factors in question are separated.
Blakeslee has found somewhat similar relations in certain of the molds. The spores in molds contain more than one nucleus, therefore it is not clear how segregation in the sense used for other cases applies here.
In the flowering plants that are hermaphroditic we have Correns' experiments, in which he crossed an hermaphroditic type of Bryonia alba with a type B. dioica in which the sexes are separate. The cross when made one way gives only females, while
172 HEREDITY AND SEX
the reciprocal cross gives males and females in equal numbers. Correns' interpretation is shown in the lower part of the next diagram.
Bryonia dioica and B. alba B. dioica $ by B. alba $ B. alba $ by B. dioica $
\ / \ /
\ / \ /
Females Females and Males
Correns' Explanation
F F B. dioica 9 {FM)—(FM) B. alba 9
(FM) — {F3I) B. alba $ F 3f B. dioica $
F(FM) female F(FM) female
M(FM) male
It is based in the first case on the assumption that the hermaphroditic condition of B. alba is recessive to the dioecious condition of B. dioica, and that the female
'f — f fctnA\c T — t1 herm. ovule
-f — H ttemv,|yqHm ^ — ft .« poWcn
f H hitrnv. "FH twmv.
^ — f male \foUctx^ ff fenvale
Fig. 88. — Diagram to illustrate G. H. Shull's results on Lychnis dioica. The symbols here used are not those used by Shull. Two types are assumed not to appear, viz. HH and Hf.
PARTHENOGENESIS 173
dioica is homozygous for the sex factor. The reciprocal cross is explained on the basis that maleness dominates femaleness. It is difficult to bring this view into line with other hypotheses of sex determination. Shull obtained as a mutant a hermaphroditic plant of Lychnis dioica. The next diagram (Fig. 88) gives the principal facts of his crosses. When a female plant is fertihzed by the pollen of the hermaphrodite, two kinds of offspring are produced — females and hermaphrodites. When the hermaphrodite is selffertilized, the same two classes are produced. When the ovule of the hermaphrodite is fertilized by the pollen from the male plant, two kinds of offspring are again produced — female and male. Shull's interpretation is too involved to give here. In the diagram the scheme is worked out on the purely arbitrary scheme that the hermaphrodite is FH, in which F is a female factor, and H a modification of it which gives hermaphroditism. This leads to the further assumption that ovule and pollen, bearing the H factor, cannot produce a plant nor can the combination / H. This scheme is only intended as a shorthand way of indicating the results, and not as an interpretation of actual conditions.
PARTHENOGENESIS
A third important condition in which the heredity of sex is involved is found in parthenogenesis.
It has long been known to biologists, that in many different species of animals and plants eggs develop without being fertilized. This is recognized as a regular method of propagation in some species. The
174 HEREDITY AND SEX
eggs are produced in the same way as are other eggs. They are produced in ovaries that have the same structure as the ovaries that give rise to ordinary eggs. Parthenogenetic eggs differ from spores, not only in their origin in an ovary, but in that they also produce polar bodies like ordinary eggs. Most, but not all, parthenogenetic eggs give rise, however, to only one polar body. Some of them at least fail to pass through the stage of synapsis, and, in consequence, they retain the full number of chromosomes.
-rt'- - -, 1
-^■'^■■§ i
Fig. 89. — Miastor, sexual male and female (to right). Three larvse
with young inside (to left) .
A few examples will bring the main facts before us.
A fly, miastor, appears in the spring of the year under two forms, male and female (Fig. 89). The eggs are fertiUzed and each produces a worm-like larva. This larva produces eggs while still in the larval stage. The eggs develop without fertilization, and produce new larvse, which repeat the process. This method of propagation goes on throughout the rest of the year until finally the adult winged flies reappear.
The bee is the most remarkable instance, for here
PARTHENOGENESIS 175
the same egg will produce, if it is fertilized, a female (queen or worker), or, if it is not fertilized, a male (drone). If the queen deposits an egg in a cell of the comb that has been built for a queen or a worker, she fertilizes the egg ; if in a drone cell, the egg is not fertilized. We need not conclude that the queen knows what she is about — the difference in shape of the drone cell may suppress the reflex, that in the other cases sets free the sperm.
The case of the bee has attracted so much attention that I may be allowed to pause for a moment to point out some of the most recent results connected with the formation of the germ-cells.
The egg produces two polar bodies — the process being completed after the sperm has entered the fertilized egg (Fig. 90). Eight chromosomes are present at each division. Eight remain in the egg (these are double chromosomes — therefore 16). The sperm brings in 8 (double) chromosomes so that the female comes to have 16 single chromosomes in her cells. There is only one kind of spermatozoon, as shown by the figure, for the first spermatocyte division is abortive — all the chromosomes passing into one cell only, and the second division gives rise to a small cell, that does not produce a spermatozoon, and a large cell that becomes a spermatozoon.
If the egg is not fertilized, it also gives off two polar bodies. It has 8 chromosomes left. The male develops with the half number. The formula for the female will be XABCD XABCD and for the male XABCD.
If the bee conforms to the ordinary type for insects,
176
HEREDITY AND SEX
we may suppose that one sex chromosome is present in the male or at least one differential factor for sex, and that it is present in all the functional spermatozoa. The female will then have two such chromosomes and come under the general scheme for insects.
A
/%r
If
x&s
l6 + ]6 = 3£^
Fig. 90. — Oogenesis and spermatogenesis in bee. Four upper figures, A-D, show formation of first {A), and second {B) polar bodies. Only inner group of chromosomes remains (C) to form egg nucleus. Entrance of sperm nucleus in D. E shows scheme of these two divisions involving eight double (82) chromosomes. F, first and second spermatocyte divisions, the first, a, b, abortive, leading to pinching off of a small cell without a nucleus, the second, c, c, leading to formation of a large (functional) and an abortive cell (above).
In the gall fly, Neuroterus lenticularis, parthenogenetic females appear early in the spring. Their eggs produce females and males — the second generation. The fertilized eggs of these females give rise the following year to the spring parthenogenetic females. Doncaster has found that each parthenogenetic female
PARTHENOGENESIS
177
produces eggs, all of which give rise to females or else to males. In connection with this fact he finds that the eggs of some females do not give off any polar bodies but retain the full number (20) of chromosomes.
10
B
10
.'.'.• 10
10
•-:•*: 2'>
ii'.
10
ili
»./
^,Sf>ernt
Fig. 91. — Illustrating chromosome cycle in Neuroterus. A, one type of spring female, whose eggs (containing 20 chromosomes) produce no polar bodies. Only sexual females result. B, the other type of spring female whose eggs form two polar bodies, leaving 10 chromosomes in egg. These eggs give rise to males. C, ripening of egg of sexual female (2d generation), and D, spermatogenesis of male (second generation).
These eggs produce sexual females (in left-hand side of Fig. 91). From the eggs of other parthenogenetic females two polar bodies are given off, and the half (10) number of chromosomes is left in the egg (see right-hand side of Fig. 91). These eggs produce males. The life cycle finds its explanation in these relations except that the origin of the two kinds of parthenogenetic females is unexplained. If we were justified in assuming that two classes of female-producing sperm are made in the male, even this point would be cleared up, for in this way the two classes of parthenogenetic females could be explained.
Fig. 92. — Life cycle of Phylloxera carycecaulis.
In another group of insects, the aphids and phylloxerans, the situation is different.
In the phylloxerans of the hickories there emerges in the spring, from a fertilized egg, a female known as the stem mother (Fig. 92). She pierces a young leaf with her proboscis, which causes a proHferation of the cells of the leaf. Eventually the leaf cells grow so fast that the stem mother is overarched in the gall that she has called forth.
Inside the gall she begins to lay her eggs.. From these eggs emerge young individuals that remain in the gall until they pass their last molt, when they become winged migrants. Externally all the migrants are alike ; but if they are dissected, it will be found that some of them have large eggs, some small eggs. But all the offspring of the same mother are of one or of the other sort.
The migrants crawl out of the opening in the gall and fly away. Alighting on other hickories, they quickly deposit their eggs. From the large eggs the sexual females emerge. They never grow any bigger than the egg from which they hatched. In fact, they have no means of feeding, and contain only one large egg with a thick coat — an egg almost as large as the female herself.
From the small eggs of the migrants, minute males are produced — ripe at their birth. They fertilize the sexual female. She then deposits her single egg on the bark of the hickory tree. From this egg (that lies dormant throughout the entire summer and following winter) there emerges next spring a female, the stem mother of a new line.
Here we find three generations in the cycle — two of which reproduce by parthenogenesis. The first parthenogenetic generation gives rise to two kinds of individuals — one makes large eggs, the other small eggs. The large eggs produce sexual females, the small eggs males.
A study of the chromosomes hri^s explained how some of tliese changes in successive generations are brought about. It has explained, for instance, how males are produced by parthenogenesis, and why the sexual egg produces only females. Let us take up the last point first.
When the spermatocytes are produced, we find, as in many other insects, that at one division a sex chromosome passes to one cell only (Fig. 93). Two classes of cells are ])roduced — one with three, one with two, chromosomes. The latter degenerates, and in consequence only the female-producing spermatozoa become fimctioiial. All fertilized eggs give rise therefore to females.
The second point that has been made out concerns the production of the male. When the small egg produces its single polar body, all of the chromosomes divide, except one, which passes out entire into the polar body. In conse(|uence the number of chromosomes left in the egg is one less than the total number. In a word, there are five chromosomes in the male, while there are six chromosomes in the female (Fig. 93). By throwing out one chromosome, the change is effected. The chromosome is the mate of the sex chromosome, that appeared as a lagging chromosome in the spermatogenesis.
In the large egg no such diminution takes place, consequently the diph^d number of chromosomes is present in tlu* female. These luiite in pairs and are reduced to three when the two polar bodies of the sexual egg are produced.
We see that by means of the chromosomes we can bring this case into line with the rest of our information bearing on the relation of the chromosomes to sex. One important point still remains to be exjilained. What causes some of the migrants to produce large eggs and others small eggs ? There must be, in all probability, two kinds of parthenogenetic eggs produced by the stem mother — or at least there must be two kinds after the single polar body has been extruded.^
Fig. 93. — Chromosoiual cycle of l\ caryoecaulis.
In another group of animals, the daphnians, parthenogenetic species occur, that, in certains respects, are like the phylloxerans ; but these species illustrate also another relation of general interest.
The fertilized winter egg produces always a female, the stem mother, which gives rise by parthenogenesis to offspring like herself, and the process may continue a long time. Each female produces one brood, then another and another. The last broods fail to develop, and this is a sign that the female has nearly reached the end of her life.
But a parthenogenetic female may produce one or two large resting eggs instead of parthenogenetic females, and the same female may at another time produce a brood of males. The large resting eggs are inclosed in a thick outer protecting case. They must be fertilized in order to develop, yet they do not develop at once, but pass through an enforced, or a resting, stage that may be shortened, if the egg is dried and then returned to water.
^ The explanation may be found in the occurrence of two types of males — one type with two sex chromosomes, the other with one — two such types were actually figured in my paper. From the type with two sex chromosomes a stem mother would be produced with four sex ehi'omosomes (two coming from the sexual egg). She would give rise to migrants with large eggs. From the type with one sex chromosome a stem mother would arise that produced small eggs with three sex chromosomes. According to whether two or one went out into the polar bodies of the small eggs, the two types of male would be reproduced.
Fig. 94. — Life cycle of Simocephalus ; successive broods in horizontal lines, successive generations in vertical lines. (After Papanicolau.)
In this life history we do not know what changes take place in the chromosomes. It has, however, often been claimed in this case that the transition from parthenogenesis to sexual reproduction is due to changes in the environment.
In fact, this is one of the stock cases cited in the older literature to show that sex is determined by external agents. It was said, that if the environment causes males to appear, then sex is determined by the environment. But as a matter of fact, in so far as changes in the environment affect this animal, they cause it to cease reproducing by parthenogenesis, and induce sexual reproduction instead. The evidence is consistent in showing that any external change that affects the mode of reproduction at all calls forth either sexual eggs or males. The machinery of parthenogenesis is switched off, and that for sexual reproduction is turned on.
The discrepancies that appear in the older accounts are probably due, as Papanicolau has shown, to different observers using females that belong to different phases of the parthenogenetic cycle. Papanicolau, starting in each case with a winter egg, finds that as successive broods are produced the color of the parthenogenetic eggs can be seen to undergo a progressive change from blue to violet. As the change progresses the chance that males and sexual eggs ('^ females ") will appear is greater. Until towards the end of the life of the individual the males and females come, as it were, of themselves (Fig. 94). If, however, individuals of successive broods are subjected to cold, it is found that while earlier broods do not respond, later ones respond more and more easily and change over to the sexual phase of the cycle.
What has just been said about the successive broods might be said equally of the first-born offspring of the successive generations, as Papanicolau's table shows (Fig. 94). Later born offspring respond more readily than do those that are historically nearer to the fertilized egg.
It seems to me that these results become a little less obscure if we suppose some substance is produced during fertilization, that is carried by successive broods and successive descendants in an ever decreasing amount. As it becomes used up, the change is indicated by the color change in the egg. When it disappears, the sexual phase comes on. Its disappearance may be hastened by cold or by starvation.
A third type, Hydatina senta (Fig. 95), an almost microscopic worm-like animal belonging to the rotifers, reproduces by parthenogenesis.
The resting egg always gives rise to a parthenogenetic female, which also reproduces by parthenogenesis. Whitney has obtained 500 generations produced in this way. But from time to time another kind of individual appears. She is externally like the parthenogenetic female, but has entirely different capacities. Her eggs may be fertilized, and if they are they become resting eggs inclosed in a hard case. The sperm enters when the eggs are immature and still in the ovary of the mother. The presence of a spermatozoon in an egg determines that the egg goes on to enlarge and to produce its thick coat. But if perchance no males are there to fertilize the eggs, this same female produces a crop of male eggs that develop into males without being fertilized at all.
There are several facts of unusual interest in the life history of hydatina, but we have occasion to consider only one of them. It has been claimed in this case also that external conditions determine the production of males. A more striking example of the erroneousness of this general conclusion would be hard to find ; for, in the first place, as we have seen, the same individual that produces males will produce out of the same eggs females if she happens to be fertilized. In the second place the older evidence which was supposed to establish the view that certain specified changes in the environment cause the production of males has been overthrown.
Fig. 95. — Life cycle of Hydatina senta.
The French zoologist, Maupas, is deserving of high praise for working out some of the most essential facts in the life cycle of hydatina, and for opening up a new field of investigation. But the evidence which he brought forward to show that by a low temperature a high production of males is caused has not been confirmed by very careful and extensive repetition of his experiments by Whitney and by A. F. Shull. The evidence that Nussbaum obtained which seemed to him to show that food conditions determined the production of males has likewise not borne the test of more recent work by Punnett, Shull, and Whitney.
It has been found, however, that the production of the sexual phase of the cycle can be suppressed so that the animals continue almost indefinitely propagating by parthenogenesis. In several ways this may be accomplished. If hydatina is kept in a concentrated solution of the food culture,- the sexual phase does not appear. The result has nothing to do with the abundance of food, for, if the food be filtered out from the fluid medium, the filtrate gives the same result. The following table given by Shull shows this very clearly.
Spring v\ AiiiK
One-fourth
One-half
Three-fourths
Undiluted
d" 2
9 ?
c? ?
9 9
d ?
? 9
d 9
? 9
cf 9
9 9
26
177
25
407
15
350
8
362
337
%oid9
12.8
5.7
4.1
2.1
0.0
Showing the number of male- and female-producers in the progeny of five sister individuals of Hydatina senta, one line being reared in spring water, the others in various concentrations of the filtrate from old food cultures.
The extent of dilution of the medium is seen to be directly in proportion to the number of sexual forms that appear. If the solution be dried and the dry substance added to ordinary water, the same end is attained.
It has not been possible to reverse the process and produce more sexual forms than are produced under ordinary conditions. This seems to mean that a change may be effected in one direction and not in the other. We cannot make a locomotive go faster than its mechanism permits, with the most favorable conditions of fuel, oil, roadbed, and engineer; but if we put in stones in place of coal, we can bring it to a standstill.
Artificial Parthenogenesis
We have now considered some of the most striking examples of natural parthenogenesis in the animal kingdom. The facts show that fertilization of the egg is not in itself essential for development. The individuals that develop from parthenogenetic eggs are as vigorous as those from eggs that have been fertihzed. We have seen that such eggs without being fertihzed are capable of producing sexual females and males. In one case, at least, we have seen how the process is accomplished.
When we review the facts of natural parthenogenesis, we find certain relations that arrest our attention.
Most parthenogenetic eggs give off only a single polar body, while fertihzed eggs without exception give off two polar bodies. This difference is clearly connected with the fact that in parthenogenetic eggs the full number or diploid number of chromosomes is retained by the egg.^ In fertilized eggs half the chromosomes are thrown out in one of the two polar bodies. The number is made good by the chromosomes brought in by the spermatozoon.
But this difference does not in the least explain natural parthenogenesis ; for we have experimental evidence to show, that an egg will develop when only half the number of chromosomes is present — one set will suffice.
There is another fact about parthenogenetic eggs that has, I beheve, been generally overlooked. Many of these eggs begin to develop into an embryo before they reach the full size of the fertilized eggs of the same species. This is true at least of the eggs of aphids, phylloxerans, daphnians, and rotifers. I interpret this
1 According to my observations on aphids and phylloxerans, the synapsis stage is omitted in parthenogenetic eggs, hence there is no union (or reduction) of the chromosomes. The omission of this stage may have something to do with parthenogenesis, although it is not evident what the relation may be.
to mean that the eggs begin their development before there has been produced over their surface a layer that in the mature egg seems to have an important influence in restraining sexual eggs from development.
This brings us at once to a consideration of what keeps sexual eggs from developing until they are fertilized.
In recent years a great variety of methods has been discovered by means of which sexual eggs can be made to develop without fertilization. This process is called artificial parthenogenesis. We owe especially to Professor Jacques Loeb the most successful accomplishment of this important feat. The discovery in his hands has led to very great advances in our understanding of the developmental process.
The chief importance of Loeb's work lies, in my opinion, not only in the production of embryos without fertilization (nature has long been conversant with such methods), but in other directions as well.
First, it has thrown light on the nature of the inhibitory process that holds back the sexual egg from developing until the sperm enters.
Second, the information gained in this way tells us something of how the sperm itself may act on the egg and start it on its course.
Third, it opens up the opportunity of studying certain problems connected with the determination of sex that can be gained in no other way.
Let me attempt briefly to elaborate some of these points.
In many eggs, perhaps in all, a membrane is produced at the surface of the egg immediately after the sperm has entered. Here we have ocular evidence that fertilization effects a change in the surface layer of the egg.
It has been shown that after this membrane is formed, the permeabihty of the egg to salts and other agents is affected and that the processes of oxidation are greatly accelerated.
In other words, the interior of the unfertilized egg is separated by means of its membrane from many things in the surrounding medium — oxygen and the salts in sea-water, for example. The egg after fertiUzation lives in a new world.
These same changes are brought about by those external agents that cause artificial parthenogenesis. But what an array of substances can cause the effect ! Many kinds of salts and of drugs, acids and alkalis, heat or cold, shaking or even sticking the surface of the egg with a minute needle.
Loeb has shown th>at development depends not only on a change in the surface of the egg, but on other changes also. Hence his most successful methods are those in which two agents are applied successively to the egg — one affects primarily the surface, the other the interior of the egg. If, for example, the eggs are placed in a solution of a fatty acid, the membrane is produced. The egg is then removed to pure sea water from which oxygen has been driven out and left there for three hours. After its return to sea water it will produce a normal embryo.
If, instead of putting the egg into water without oxygen, a hypertonic solution of salts is used (50 cc. of sea water plus 8 cc. of 23^^ NaCl), the development may be carried through.
Loeb concludes that the oxidations set up in the egg by a change in its outer surface affect the egg itself injuriously ; and unless they are removed or the effects are counterbalanced by some other change (as when a hypertonic solution is used) the egg goes to pieces. Hence he believes that the sperm has a double role in fertilization. First it changes the surface layer and increases in consequence the oxidations in the egg ; second, the sperm brings into the egg some substance that counteracts poison produced by the oxidation itself.
This is what fertilization accomplishes from a physiological point of view. In addition, we have seen that fertilization brings into the egg certain materials whose presence affects the characters of the individuals that develop from it. This is what fertilization does from the point of view of the student of heredity.
Let us turn for a moment, in conclusion, to the question of sex of animals that come from artificially parthenogenetic eggs.
In natural parthenogenesis such eggs may develop into males, sexual females, or parthenogenetic females.
But in artificial parthenogenesis the egg has already undergone reduction in its chromosomes and is represented by half of the female formula as far as the chromosomes are concerned. The half formula will be XABC for the type with homozygous female. Since the egg has one X it may be expected to become a male, but if sex is a relation of X to ABC, one cannot be certain that it might not be a female.
In cases where the female is heterozygous for the sex factor, as in birds and some sea urchins, the formula for the female would be XABCD — YABCD and for the male YABCD — YABCD. There would be two types of eggs, XABCD and YABCD. The former might be expected to produce a female, the latter probably a male if such eggs were incited artificially to develop.
Concerning the sex of the embryos so far produced by artificial parthenogenesis, we know of only two cases. These two cases are Delages' result for the sea urchin, in which he got one male, and Loeb's and Bancroft's case for the frog, in which they believe that the two young obtained were females.
What to expect on theoretical grounds is uncertain. We have only two facts that bear on the question. In the parthenogenetic eggs of the aphid, with the formula XABC ABC we get a male. In the case of the bee the formula is XABC, which also gives a male. All else is hypothetical and premature, but if these two formulae are correct, it appears that one X gives a male and that maleness is not due to a quantitative relation between X and one or two sets of the other chromosomes. It is the quantity of something in X, not the relation of this to the rest of the chromosomes.
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
Cite this page: Hill, M.A. (2024, April 19) Embryology Heredity and Sex (1913) 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Heredity_and_Sex_(1913)_6
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