FIGURE 7-3 Early human embryos. Ovulation ages: A. 19 days (presomite). B. 21 days (7 somites). C. 22 days (1o7 somites). (After drawings and models in the Carnegie Institute.) he conceptus is termed an embryo at the beginning of the third week after ovulation and fertilization. Primitive chorionic villi form, and this coincides with the expected day of menses. he embryonic period, during which time organogenesis takes place, lasts 6 weeks. It begins the third week from the LMP through the eighth week. The embryonic disc is well deined, and most pregnancy tests that measure human chorionic
FIGURE 7-4 Three-to four-week-old embryos. A, B. Dorsal views of embryos during 22 to 23 days of development showing 8 and 12 somites, gonadotropin (hCG) become positive by this time. As shown in Figure 7-3, the body stalk is now diferentiated. There are villous cores in which angioblastic chorionic mesoderm can be distinguished and a true intervillous space that contains maternal blood.
During the third week, fetal blood vessels in the chorionic villi appear. In the fourth week, a cardiovascular system has formed (Fig. 7-4). hereby, a true circulation is established respectively. (-E. Lateral views of embryos during 24 to 28 days, showing 16, 27, and 33 somites, respectively. (Redrawn from Moore KL: The Developing Human: Clinically Oriented Embryology, 4th ed. Philadelphia, Saunders, 1988.)
FIGURE 7-5 Embyro photographs. A. Dorsal view of an embryo at 24 to 26 days and corresponding to Figure 7-4C. B. Lateral view of an embryo at 28 days and corresponding to Figure 7-40. C. Lateral view of embryofetus at 56 days, which marks the end of the embryonic period and the beginning of the fetal period. The liver is within the white, halo circle. (From Werth B, Tsiaras A: From Conception to Birth:
A Life Unfolds. New York, Doubleday, 2002.) both within the embryo and between the embryo and the chorionic villi. Partitioning of the primitive heart begins. Also in the fourth week, the neural plate forms, and it subsequently folds to form the neural tube. By the end of the ifth menstrual week, the chorionic sac measures approximately 1 cm in diameter. he embryo is 3 mm long and can be measured sonographically. Arm and leg buds have developed, and the amnion is beginning to ensheathe the body stalk, which thereafter becomes the umbilical cord. At the end of the sixth week, the embryo measures approximately 9 mm long, and the neural tube has closed (Fig. 7-5). Cardiac motion is almost always discern able sonographically (Fig. 7-6). The cranial end of the neural tube closes by 38 days from the LMP, and the caudal end closes by 40 days. Thus, the neural tube has closed by the end of the sixth week. And by the end of the eighth week, the crownrump length approximates 22 mm. Fingers and toes are present, and the arms bend at the elbows. The upper lip is complete, and the external ears form definitive elevations on either side of the head. hree-dimensional images and videos of human embryos from the MultiDimensional Human Embryo project are found at: http://embryo.soad. umich.edu/index.html.
FIGURE 7-6 A. This image of a 6-week, 4-day embryo depicts measurement of the crown-rump length, which is 7.4 mm at this gestational age. B. Despite the early gestational age, M-mode imaging readily demonstrates embryonic cardiac activity. The heart rate in this image is 124 beats per minute.