58 



EMBRYOLOGY 



length of the fetus, i. e., about 50 cm., but it may be greatly diminished or increased. 

 The rudiment of the umbilical cord is represented by the tissue -which connects 

 the rapidly growing embryo with the extra-embryonic area of the ovum. Included 

 in this tissue are the body-stalk and the vitelline duct the former containing the 

 allantoic diverticulum and the umbilical vessels, the latter forming the communica- 

 tion between the digestive tube and the yolk-sac. The body-stalk is the posterior 

 segment of the embryonic area, and is attached to the chorion. It consists of a plate 

 of mesoderm covered by thickened ectoderm on which a trace of the neural groove 

 can be seen, indicating its continuity with the embryo. Running through its 

 mesoderm are the two umbilical arteries and the two umbilical veins, together with 

 the canal of the allantois the last being lined by entoderm (Fig. 31). Its dorsal 

 surface is covered by the amnion, while its ventral surface is bounded by the extra- 

 embryonic celom, and is in contact with the vitelline duct and yolk-sac. With 

 the rapid elongation of the embryo and the formation of the tail fold, the body 

 stalk comes to lie on the ventral surface of the embryo (Figs. 27 and 28), where 



Somatic mesoderm 



Splanchnic 

 mesoderm 

 Entoderm 



Vitelline veins 



Amniotic cavity 

 Amnion 

 Neural groove 



Body-stalk 



FIG. 31. Model of human embryo 1.3 mm. long. (After Eternod.) 



its mesoderm blends with that of the yolk-sac and the vitelline duct. The lateral 

 leaves of somatopleure then grow round on each side, and, meeting on the ventral 

 aspect of the allantois, enclose the vitelline duct and vessels, together with a part 

 of the extra-embryonic celom; the latter is ultimately obliterated. The cord is 

 covered by a layer of ectoderm which is continuous with that of the amnion, and 

 its various constitutents are enveloped by embryonic gelatinous tissue, jelly of 

 Wharton. The vitelline vessels and duct, together with the right umbilical vein, 

 undergo atrophy and disappear; and thus the cord, at birth, contains a pair of 

 umbilical arteries and one (the left) umbilical vein. 



Implantation or Imbedding of the Ovum. As described (page 44), fertilization 

 of the ovum occurs in the lateral or ampullary end of the uterine tube and is 

 immediately followed by segmentation. On reaching the cavity of the uterus the 

 segmented ovum adheres like a parasite to the uterine mucous membrane, destroys 

 the epithelium over the area of contact, and excavates for itself a cavity in the 

 mucous membrane in which it becomes imbedded. In the ovum described by 



