Male advancement in prenatal hand development.

Male advancement in the developing hand was evidenced by 66 grossly and microscopically normal embryos in the 15-75 mm crown-rump range. Male advancement was particularly pronounced in the younger (1 530 mm) embryos, both in the proximal hand region including the round bones of the wrist and in the distal hand region, comprising the metacarpals and phalanges. Thirty-four additional embryos, abnormal in implantation or development or representing spontaneous abortion similarly evidenced male advancement in the early hand skeleton In most aspects of human development, from before birth through the end of adolescence, the female is advanced over the male, by from 4 to 20% and more. This is true for ossification, as measured by the radiographic appearance of the nuclei of the round bones and epiphyses (Garn et al., '67). This is true for epiphyseal union, as seen in the hand, foot, elbow, knee, shoulder and hip (Noback, '54; Garn et al., '61). This is also true for the calcification, movement and clinical eruption of the permanent teeth, to an extent exceeding differences attributable to race (Garn et al., '58; Garn et al., '65). Female advancement in the development of the secondary sexual characteristics and in the onset of gametogenesis are well known in human biology and accepted as fact in law. For the early prenatal period, however, during the first trimester of gestation, female advancement is not known as a fact, possibly because of the lack of comparative sex-specific embryological data. Standard works are vague on this subject, from Arey's textbook to the more specialized reports of Streeter ('49), Gray et al. ('57), and Blechschmidt ('69). Indeed, in our previously-published studies on early prenatal development of the primary (deciduous) dentition, we have found male advancement rather than the female advancement we had earlier anticipated, (Burdi et al., '70; Garn and Burdi, '71). We therefore raise the question as to whether the deciduous teeth are unique, AM. J . PKYS. ANTHROP., 41: 353-360. in suggesting male rather than female advancement in the 15-75 mm crownrump range, or whether male advancement is a developmental characteristic of the entire first trimester. Accordingly, therefore, we have turned our attention to the developing hand, including both the wrist area and the metacarpals and phalanges, in a prenatal developmental investigation patterned after now-familiar postnatal developmental models (cf. Garn et al., '67). METHODS AND MATERIALS This comparative study of early prenatal hand development is based upon two series of human embryos, all in the 15-75 mm crown-rump length range and all clearly identifiable as to sex. The first series, upon which this paper primarily depends, comprises some 66 embryos, 40 of them male and 26 female, all of them from documented normal (uterine) implantations, and all apparently normal by both gross inspection and after examination of the histological sections. The second series, carefully separated from the first, consisted of an additional group of 34 embryos of similar crown-rump range and similarly identified as to sex, but excluded from the normal group because of abnormalities of implantation (ectopic pregnancies), or because of spontaneous abortion, or because of neural arch defects. Sexing was accomplished by histological