Paper - Twins, normal and abnormal
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Johnson HH. Twins, normal and abnormal. (1939) NY Acad. Sci. 1(4):
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Twins, Normal and Abnormal
Doctor H. Herbert Johnson, College of the City of New York:
(This lecture Jan 9, 1939 was illustrated by lantern slides, clay models, and specimens.)
In spite of the familiarity of the topic, it is obvious that much is still to be learned concerning the exact origin in embryo of human twins and partial twins, while at the same time there exist widespread misconceptions regarding the phenomenon of twinning. A review of current ideas bearing upon the problem seems timely, especially since opportunity is given to illustrate some points with preserved specimens of unusual interest. No attempt is made here to present very original points of view on the matter, and the discussion is limited to well known theories of special interest to the embryologist.
Dizygotic (“fraternal” or two-egg) human twins theoretically may result from any one of three possibilities: 1. More or less simultaneous ovulation from both ovaries; 2. Ovulation, at about the same time, of more than one follicle in a single ovary; 3. Ovulation of a single follicle containing two or more ova. Evidence is presented that such biovate or polyovate follicles are much more frequent than is generally believed. The exhib 1 No meetings were held in January by the Sections of Geology and Anthropology.
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its include examples from the pig, rabbit, dog, cat and monkey. The view of Maximov that such follicles are characteristic of young females, is not supported by this investigation.
It is now quite generally accepted that about one fourth of all twins are monozygotic (“identical” or single-egg) but no single line of evidence gives irrefutable proof that monozygotic twins exist. The various sources of evidence which collectively tend to establish proof of single-egg twins are reviewed in the recent book by Newman et al.‘
Statistical evidence is gleaned from the fact that same-sexed twins occur more frequently than opposite-sexed twins, whereas the genetic expectation is a 1: 1 ratio of same-sexed to oppositesexed pairs. The weakness in this line of evidence when considered alone lies in the assumption that opposite-sexed twins are equally viable in utero as same-sexed pairs, and are in no way subject to environmental sex changing disturbances such as have been demonstrated experimentally in other higher vertebrates.
Very important is the fact that conjoined twins are always of the same sex, and so are twins born in a single chorion. Until 1928 it was generally believed that all monozygotic twins develop in a common chorionic envelope. In 1927 Komai stated: “Apart from the examination of the placenta and foetal membranes at birth, there is no safe criterion of distinguishing the identical twins from the fraternal twins.””
The fact that identical twins are so frequently monochorial refutes a widespread dogma that such individuals originate by a splitting apart of the blastomeres at ﬁrst cleavage. This point of View results from a misguided application to the human egg of the classical experimental results obtained on other forms. Chabry, Herbst, Driesch, Wilson et al. obtained more or less “normal” development of certain marine invertebrates and of Amphioxus from single blastomeres isolated experimentally during early cleavage. It should be emphasized that
1 Newman, Freeman, and Holzinger. “Twins: A Study of Heredity and Environment.” University of Chicago Press, 1937. 2Komai, Taku. Science, 65: 280. 1927.
these eggs do not self-isolate their own blastomeres. There may be no theoretical reason why the human ovum, or that of any mammal, might not divide at a stage in early cleavage into entirely separated blastomeres each capable of producing a complete body. The presence of the single chorionic envelope, however, shows that this is not usually the case. Since the chorion in the mammal embryo is developed from the outer layer of cells of the blastula and the embryo results from a clump of more centrally placed cells, it is obvious that the real division of the body in such cases results from the separation of this inner cell mass at a comparatively advanced stage of blastula formation. Such duplicated inner cell masses have been described in the armadillo by Patterson, the sheep by Assheton, and the.human by Arey.“
Since 1928 some evidence has accumulated to show that perhaps a few monozygotic twins are indeed dichorial. The case of identical triplets described by Komai and Fukuoko,‘ 1931, is not altogether conclusive when considered alone. Two bodies were produced in a chorion, and the third was in a separate chorion attached to the ﬁrst. Komai interpreted this as a fusion of two separate chorions, but it could almost equally be called a faulty separation of one. Other evidence for dichorial identical twinning is presented by Curtius° and Lassen.’ It is still unknown whether these cases really represent a separation of early blastomeres, or a constriction of the chorion at a much later stage, perhaps while invasion of the uterine wall is taking place.
Duplication of body parts occurs in an extensive series ranging from duplication of a single digit or the palm of the hand to duplication of the entire limb, the head, or practically the entire body (the conjoined, or “Siamese” twin). Conjoined twins apparently arise later in gastrulation than identical twins. The dissimilarity of the individuals has been explained in a view advanced by Newman.‘ One is developed entirely from that part of the embryonic disc previously differentiated as left, and the other from the corresponding rudiment already determined as right. Each rudiment restores the missing portion in mirror-image symmetry to itself. Thus the two bodies differ to about the same degree as is frequently seen in the left and right sides of an ordinary person.
3 Arey, L. B. Anat. Rec., 23, 1922.
4 Arey, L. B. “Developmental Anatomy,” 3rd Ed., p. 154, 1934. W. B. Saunders, Publisher.
5 Komai, Taku and Gore Fukuoka. Jour. of Hered., 22, 1931.
6 Curtius, F. Zeitschrift fiir Konstitutionslehre, 13, 1928.
7 M. T. Lassen. Archiv fiir Gynakologie, 147, 1931.
After reviewing various theories as to the basic causative factor leading to embryonic axial duplication, the theory of developmental arrest, advanced especially by Stockard,° still seems to be, in a general way, the best interpretation. The Spemann school has shown that the portion of the embryonic disc capable of being organized to produce a head is a considerably larger ﬁeld than is actually necessary to produce a head. The center is the more responsive portion of this ﬁeld. If a developmental arrest occurs precisely at the time that evocatory stimulation, capable of organizing a head, is expected, the center of the ﬁeld is retarded and loses its power of development. If the retarding factor then ceases to operate, some power of organization still is retained by the more lateral portions of the head-ﬁeld, and these separately begin development, thus forming a duplicated head. Similar arrest, occurring at the critical moment at which gastrulation should begin, might similarly result in the appearance of two gastrulation points on a single embryonic disc. The degree of mechanical and physiological separation of these centers is in direct ratio to the degree of separation exhibited by the resulting twins.
Specimens exhibited included a foetal pig with duplicated snout and awe, and three eyes; a human foetus of ﬁve months with two complete heads, tour shoulders, and three arms; a human octopus. with two nearly complete bodies joined at the pelvis. These specimens and others shown. are part of the collection of the Division of Embryology of the College of the City of New York.
8 Newman, H. H. Journ. of He:-ed., 22, 1931. 9 Stockard, C. R. Amer. Jour. Anat., 28, 1921.
Cite this page: Hill, M.A. (2020, June 3) Embryology Paper - Twins, normal and abnormal. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Twins,_normal_and_abnormal
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