380 THE NEW INDIVIDUAL Part IV 



in addition to the yolk sac and allantois also holds the large blood vessels 

 that connect the embryo with the placenta (Fig. 19.18). 



Human Embryo 



First Days of Life. DifTerent as they may be later, animals greatly resemble 

 one another in the earliest part of their lives (Fig. 38.7). In their youngest 

 stages, rabbits, monkeys, and men look very much alike, though their chromo- 

 somes soon tell a different story. In recent years, the microscopic living 

 embryos of mice, rabbits, and monkeys have been removed from the maternal 

 oviducts and uteri, placed in salt solution at body temperature, and photo- 

 graphed in still and motion pictures (Fig. 19.16). The youngest human em- 

 bryos yet seen, including a 2-celled one, have been removed from the oviducts 

 and uteri of persons undergoing operations (Fig. 19.17). The fertilization 

 of the human egg on a microscope slide has also been photographed. 



Implantation in the Uterine Wall. With the help of cilia and contractions 

 of muscles in the wall of the oviduct the human embryo is rolled into the 

 uterus. By the time it arrives there, or soon after that, it reaches the blastocyst 

 stage (Fig. 18.14). This is an almost microscopic sphere, its wall a thin layer 

 of cells (trophoblast), mostly chorion, that contains fluid and a knot of 

 cells, the embryo. In this stage it is presumed to be about 4 to 5 days old, 

 counting from the time that the egg was probably fertilized. Within a day or 

 two, the blastocyst sticks to the lining of the uterus, and then sinks into it, 

 evidently through the effect of its own secretions upon the cells about it. In 

 the meantime, delicate fingerlike processes, the villi, grow out from the 

 surface of the little sphere into the wall of the uterus, like roots into soil 

 (Figs. 18.14, 19.15). There they are surrounded by blood from the uterine 

 capillaries whose walls have been broken during this process of implantation. 

 Thus the embryo's source of supplies is at once established. At first, all ex- 

 change of water and food and gases is by absorption through the membranes 

 and body of the embryo. Later, the blood vascular system develops and the 

 embryo's own blood transports materials always to and from the villi extend- 

 ing into the mother's blood. The two kinds of blood never mix. 



Placenta. The placenta is a temporary organ formed from parts of two 

 individuals of different generations, the mother and her unborn young. Its 

 maternal part (or decidua) is an elaborate development of the inner layers 

 of the uterine wall. Its embryonic part is a specialized region of the chorion. 

 In the human placenta there are open spaces or sinuses between these two 

 parts into which maternal blood flows from uterine arteries that were first 

 broken during the implantation of the embryo. This blood is constantly 

 changed as it flows from the uterine arteries and slowly returns to the 

 uterine veins. Minute richly branched villi from the embryonic placenta dip 

 into this reservoir of blood (Fig. 19.15). Within each fingerlike villus are 



