Paper - Direct proof of the monozygotic origin of human identical twins (1922)

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Arey LB. Direct proof of the monozygotic origin of human identical twins. (1922) Anat. Rec. 22(4): 245-252.

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This historic 1922 paper by Arey described monozygotic twinning in humans.

See also by this author: Arey LB. Chorionic fusion and augmented twinning in the human tube. (1922) Anat. Rec. 22(4): 253-262.

See the comment on this paper by Frederic Lewis.

Lewis FT. The futility of the human yolk sac. (1922) Science 55 (No. 1427): 478.

Historic Embryology - Twinning 
1915 Twin embryos 17-19 somites | 1916 Conjoined Twins | 1922 Monozygotic origin identical twins | 1927 Separate amnions | 1942 Twin chorion | 1955 Twins and multiple birth
Carnegie Embryos: 8505a 8505b 7170a 7545 9009a and b 9123 5542B 5935A 5621A

Modern Notes twinning

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Direct Proof Of The Monozygotic Origin Of Human Identical Twins

Leslie Brainerd Arey (1891-1988)
Leslie Brainerd Arey (1891-1988)

Leslie B. Arey

One Plate (Six figures) Contribution no. 90. (1922)

The commonest seat of ectopic pregnancy is the uterine tube. When such tubal gestations are plural, they may be either unilateral or bilateral in position. The former type, that is, the presence of more than one fetus in the same tube, is somewhat the rarer; a careful examination of the literature (’22 a) has yielded thirty—eight positive or authentic cases, eight probable or presumptive cases, and four possible but doubtful cases. To the positive list I have there added two new cases of monochorial twins, each unique of its kind. Detailed descriptions of these specimens, summaries of all the other cases, together with the general conclusions drawn from an analysis of the anatomical and clinical data afforded, may be found elsewhere (’22 a, ’22 b, ’22 c). This note is confined to the presentation of certain features of peculiar embryological interest and significance.

The first of the two new specimens} consisted of a single chorionic sac which contained twin embryos, each 12.3 mm. long (fig. 1). There is a common yolk sac from which distinct yolk stalks arise near together and pass to their respective umbilical cords. The cut stumps of these yolk stalks and their separate origin from the sac are clearly shown in figures 2 and 3.

2 This twin specimen belongs in the collection of the Carnegie Institution, where it is listed as no. Template:CE1126. Acknowledgment is due the director, Dr. George L. Streeter, for permission to describe it. He has also furnished the accompanying photographs (figs. 1 to 3).

There has been no real doubt that human homologous or identical twins come from the division of a single egg. Many facts point incontestably to this conclusion; among such are their morphological duplicity, their occasional conjoining as ‘double monsters,’ and a common chorion.3 Direct corroboration is furnished by the known manner of quadruplet development in the Texas armadillo, as well as by experiments and observations upon the lower Vertebrates and invertebrates. Yet this. specimen of human twins demonstrates for the first time their actual origin from a single yolk sac, and hence from a single ovum. Schwalbe’s (’06) well-known reconstructions, based upon the Spee 1.54-mm. embryo, are purely hypothetical. Beside the present case, the only other illustrative specimens among mammals are the normal developmental stages of the Texas armadillo (Newman and Patterson, ’10) and Assheton’s (’98) single case of a sheep’s blastodermic vesicle which bore twin germinal areas.

The second new twin specimen is in some respects even more interesting. Within the single chorion were twin embryos of 11.5 and 12 mm. (fig. 4). Each had its individual umbilical cord; these were attached to the chorionic wall, a quadrant’s distance apart. Adherent to the amnion of one embryo (at the left in fig. 4) was a yolk sac of normal size; its cut stump may be seen in the photograph. The other embryo has no yolk sac. To make certain of this, both umbilical cords were sectioned throughout their lengths. That of the embryo at the left was typical for this age (fig. 5). The yolk stalk with its vitelline vessels lies in a coelomic extension; the level of the section here reproduced is too far distad to include the allantois. The cord of the embryo at the right is without microscopic trace of yolk sac or stalk, although the remaining structures are normal (fig. 6).

3 In another publication (’22 c) it will be shown that a common chorion is not infallible proof of the monozygotic origin of human twins.

Certain deductions may also be made from this specimen. Mammalian monozygotic twins do not arise by the separation of early blastomeres, as too often has been assumed; on the contrary, the common chorion which all twins of this sort possess proves at once that separation must be subsequent to the period when the cleavage group is differentiated into an inner cell mass and an outer shell of trophectoderm. That identical monochorionic twins arise by the fusion of separate ova need not seriously be considered, although it is apparent that some double ovum (fraternal) twins do become monochorial, in this way (’22 c). There is considerable evidence to support the belief that the twinning impulse occurs relatively late, at about the moment of gastrulation, which in mammals corresponds to the time of the formation of a primitive streak; the process, then, is simply one of double gastrulation. Studies on the development of the Texas armadillo (Newman and Patterson, ’10) and Stockard’s (’21) basic experiments and conclusions on the production of twins both substantiate this belief.

Exactly how determinate the mammalian ovum is in its development and how rigidly formative stuffs are localized are matters of speculation. The manner of fission into the two embryonic masses was presumably such that one received all, or essentially all, the cells destined to form a yolk sac; this again confirms the idea of a late moment for twinning when other cells had passed the stage of fundamental multiple potentialities.

The total absence of a yolk sac in one embryo which is otherwise normal in every way further demonstrates conclusively that this organ is not essential to the growth of an embryo or to the proper differentiation of its parts; indeed, the embryo in question is slightly larger than its twin, whereas both correspond closely in size to the norm for that menstrual age.

The embryo without a yolk sac is of further interest in relation to the ingrowth doctrine of vasculogenesis. In so far as such evidence is trustworthy, it supports rather the local-origin view, now generally acknowledged. That tiny vascular anlages of yolk-sac ancestry actually existed and at an earlier moment helped form primitive vessels by extension into the embryo is, of course, conceivable; also the theoretical possibility of vascular ingrowths from the early anlages in the body stalk are not excluded because of the characteristic and necessary nutritive relations of the mammalian ovum. Fortunately, the fundamental truth of the local-origin doctrine rests on observations more rigorously controlled than this natural experiment which, however, was performed as perfectly as ever may be expected in a higher mammal.

In general, the yolk sac of man and the higher mammals appears to be an unnecessary vestige. In the earliest human embryos known, when it might be of real use, it is a simple entodermal sac containing masses of coagulum; growth to a conspicuous size is attained relatively late, long after adequate nutritional relations with the mother have been thoroughly established.

It is a great coincidence that both specimens of so rare a type should show features of such singular value and interest.


Human identical or homologous twins for the first time are actually shown to originate from a single ovum.

The human yolk sac is a vestige unessential to growth or differentiation (including vasculogenesis).

The local-origin doctrine of vasculogenesis is supported within the limits of experimental possibility on mammals.

Literature Cited

AREY, L. B. 1922 a. The complete data of tubal twin pregnancy. (In preparation.)

1922 b Tubal twins and tubal pregnancy. Surg. Gynec., and Obstet. (In press.)
1922 c Chorionic fusion and augmented twinning in the human tube. Anat. Rec., vol. 23, pp. 253-262.

ASSHETON, R. 1898 An account of a blastodermic vesicle of the sheep of the seventh day, with twin germinal areas. Jour. Anat., vol. 32, pp. 362-370.

NEWMAN, H. H., AND PATTERSON, J. T. 1910 Development of the nine-banded armadillo, etc. Jour. Morph., vol. 21, pp. 359-444.

SCHWALBE, E. 1906 Die Morphologie der Missbildungen des Menschen. 1. Teil. fischer, Jena.

STOCKARD, G. R. 1921 Developmental rate and structural expression: etc., Am. Jour. Anat., vol. 28, pp. 115-278.

Plate 1


1 Photograph of 12.3-mm. human tubal twin embryos attached to a common yolk sac. X 2.2.

2 and 3 Additional photographs of the common yolk sac shown in figure 1. The origin of the separate yolk stalks is apparent. X 2.2.

4 Photograph of a pregnant human uterine tube. Part of the wall has been removed to show the twin embryos in place. The single yolk sac belongs to the 11.5-mm. embryo at the left. The 12-mm. embryo at the right lacks both yolk sac and stalk. X 1.7.

5 Photomicrograph of a section through the umbilical cord of the embryo at the left in figure 4. The yolk stalk lies in a coelomic extension. The level is too far distad to include the allantois. X 23.

6 Photomicrograph of a section through the umbilical cord of the embryo at the right in figure 4. There is no trace of a yolk stalk. The delicate tube below is the allantois. X 23.

Cite this page: Hill, M.A. (2024, February 28) Embryology Paper - Direct proof of the monozygotic origin of human identical twins (1922). Retrieved from

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