252 THE CONTINUITY OF THE RACE 



Embryonic Development and the Birth of the Child 



When it is first implanted in the uterine lining, the embryo is micro- 

 scopically small. It is soon enclosed by the growing maternal tissues. In 

 a few weeks the embryo and its membranes have become large enough 

 to cause the part of the wall containing them to bulge out into the cavity 

 of the uterus and shortly thereafter to fill this cavity completely. Further 

 growth of the embryo is accompanied by enlargement of the uterus, until 

 at full term the latter has become about 400 times its original volume and 

 occupies most of the space in the now greatly protuberant abdomen. 



At an early stage of its development the embryo gives off an outer 

 spherical layer of cells, which becomes the outermost of the embryonic 

 membranes, the chorion. The embryo remains attached to the chorion at 

 one point by the body stalk, which later becomes the umbilical cord, and is 

 enclosed in a fluid-filled amniotic sac within the chorion. Blood vessels 

 forming in the region of the body stalk grow into the chorion and there 

 come to lie close to the blood-rich maternal tissues of the uterine wall. A 

 circular area around the region of attachment of the body stalk develops 

 into a richly vascular, pancake-shaped mass of intimately connected 

 embryonic and maternal tissues, called the placenta. In the placenta 

 there is no actual mingling of the blood streams of mother and child, but 

 they are separated only by thin membranes through which dissolved 

 substances can easily pass by diffusion. Oxygen, food substances, hor- 



D, after 21 days of development. The embryo is at the right; the elongated yolk sac, at 

 the upper left; and the umbilical cord that connected the embryo to the placenta, at the 

 lower left. The embryo is now 0.117 inch long. Note the series of somites or segments, the 

 slightly curved brain region at the upper end, and the saclike heart below the brain and 

 between the yolk sac and the body of the embryo. The heart has already begun to beat 

 faintly. 



E, the embryo enclosed in its extraembryonic sac, the chorion. This specimen is twenty- 

 eight days old and about an inch in diameter. The many long, slender projections (villi) 

 of the chorion grow into the walls of the mother's uterus to form the placenta through which 

 oxygen and food, carbon dioxide and wastes are exchanged. 



F, this is the same embryo shown in E, with the chorion cut open to show the embryo 

 floating in the cushioning embryonic fluid. The embryo is the large C-shaped object in 

 and above the center of the cavity. Compare with G. 



G, a four-weeks-old embryo removed from the chorion. This embryo is viewed from the 

 left, that in F, from the right. Note the gill furrows just back (to the right) of the enlarged 

 anterior end of the brain and separated by the gill arches or bars, the posterior curved tail, 

 and the left posterior limb bud (leg) just anterior to the tail. The left anterior limb bud 

 (arm) is barely visible to the right of the heart, which is the enlarged structure that pro- 

 jects outside of the body of the embryo anterior to the neck of the yolk sac. 



H, a forty-day-old embryo inside a thin, intact membrane, the amnion, which in turn 

 lies in the opened chorion. A third small extraembryonic membranous sac, the allantois, is 

 seen as a small sphere just outside of the amnion. Both amnion and chorion are filled with 

 fluids which protect the developing embryo. In the embryo itself the finger lobes of the 

 developing left hand are plainly visible, as well as the now larger leg buds, the enlarged head, 

 and the developing eye. 



I, an eight-weeks-old embryo. The developing head, eye, ear, nose, and mouth can now 

 easily be recognized, and the fingers and toes of the hands and feet are assuming their human 

 form. Note the forming ribs. The entire skeleton is cartilaginous at this stage. 



