THE UMBILICAL CORD. . 115 



trophoderm. Second, to secure the attachment of the ovum to the wall of the 

 uterus. This preliminary attachment is called the implantation of the ovum. In 

 some cases the trophoderm is developed very early over the surface of the ovum 

 (Fig. 74), appearing almost as soon as the stage of the blastodermic vesicle is 

 reached, and while the vesicle is very small. In such cases the ovum creates a 

 space for itself by dissolving away the epithelium and connective tissue at a small 

 spot on the uterine surface, making a cavity in which the ovum lodges. In other 

 cases the trophoderm is developed later and does not appear over the whole of the 

 blastodermic vesicle. The area over which it exists in such cases is called the 

 placental area (compare pages 127 and 179). The trophoderm in these forms 

 unites very closely indeed with the surface of the uterus (Fig. 37, Tro) and the 

 uterine tissues undergo degeneration and resorption. We may regard as the first 

 step toward the production of the placenta proper the disappearance of the tropho- 

 derm. Our knowledge of its disappearance is incomplete, but it is probable that 

 it is due to a transformation of the cells of the trophoderm, associated with con- 

 temporaneous modifications of the chorionic membrane, of which the general result 

 may be said to be formation of the chorionic villi which constitute the fetal pro- 

 tion of the placenta. The modified trophoderm cells are supposed to enter into 

 the formation of the ectodermal covering of these villi. 



The Umbilical Cord. 



The umbilical cord may be best defined as the tissues connecting the body 

 proper of the embryo with the amnion. It accordingly includes a portion of the 

 body-stalk and a certain extent of the body-wall or somatopleure. In early stages 

 we can hardly speak of an umbilical cord, because the amnion arises close to the 

 embryo (Fig. 83). As development progresses the body-stalk lengthens out and the 

 amnion arising from it recedes farther and farther from the embryo, this recession 

 being assisted by a growth of the somatopleure which leads to the formation of the 

 umbilical cord proper. By this means a tubular structure is produced, the cavity 

 of the tube being a prolongation of the coelom of the embryo. During the first 

 develbpment of the umbilical cord the neck of the yolk-sac becomes constricted and 

 very much lengthened out, forming the yolk or vitelline stalk. The yolk-stalk 

 springs within the embryo from the wall of the intestine, runs through the coelom 

 of the umbilical cord, and makes its exit beyond the amnion, as shown in figure 

 102. The yolk-sac proper still occupies its original position between the amnion 

 and chorion. The student should note carefully that the umbilical cord is never 

 covered by the amnion, for it has unfortunately been often stated that it is so 

 covered. Ah idea of the relations can be gathered from cross-sections of the cord 

 (Fig. 66). The ccelom, Cw, is a large cavity and contains the yolk-stalk, Y, with 

 two blood-vessels, but with its entodermal cavity entirely obliterated. Above the 

 body-cavity is the duct of the allantois, All, lined by entodermal epithelium, and in 



