Morphogenesis and histogenesis of the facial muscles is described in human embryos and fetuses 4.2-360.0 mm (crown-rump length). The microscopic study was performed on 50 specimens that had been variously serially-sectioned and stained. Graphic or wax reconstructions were made from transverse serial sections at 11 representative ages. Three late fetuses and a term infant were studied grossly.Second branchial arch mesenchyme in early embryos (4.2-6.5 mm) becomes increasingly dense, but is not subdivided into distinct premuscle masses. By 8-20 mm, sheet-like collections (laminae) of premyoblasts and early myoblasts extend from the superficial part of the arch into the temporal, occipital, cervical and mandibular regions. Premuscle condensations deep in the arch become the stapedius, posterior digastric and stylohyoid muscles. The infraorbital lamina and the occipital platysma appear by 20-23 mm. The superficial muscles differentiate rapidly between 26 and 37 mm. Most are composed of myoblasts at 41 mm. By 80 mm, all the muscles con-

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Differences in human chorionic gonadotrophin (HCG), average chorionic sac diameter, embryo/fetus crown-rump length and biparietal diameter were determined in 107 singleton pregnancies delivered after 34 weeks, whose post-insemination age was precisely known as a result of in-vitro fertilization (n = 28) or gamete intra-Fallopian transfer (n = 79). Crown-rump lengths were interpreted in relation to the developmental stage of the embryo. A 7-fold to 10-fold difference in HCG levels was observed on post-insemination days 13-16. A 2-fold to 3-fold difference occurred in average chorionic sac diameter on days 25-36. A 2-fold difference occurred in crown-rump length on seven of 10 days prior to day 44. Biparietal diameter differed by no more than 42% from day 57-68 and 20% after day 68. Chorionic sac diameter, crown-rump length, and biparietal diameter were also related to newborn weight. The results suggest firstly that marked differences occur in the rate of early human development, secondly, that the differences occur prior to day 27 when observable cardiac activity begins, and thirdly, that the differences are minimized after day 68 when the embryonic period of development is completed.

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