3 IO EMBRYOLOGY 



neck, the body or any of the members of the foetus ; and this can be 

 accounted for only by the movements of the foetus in utero. 



The external covering of the cord is a process of the amnion ; and 

 as it extends over the vessels, it includes a gelatinous substance (the 

 gelatin of Wharton) which surrounds the vessels and protects them from 

 compression. This gelatinous substance is identical with the so-called 

 membrana intermedia, or the substance included between the amnion 

 and the chorion. The entire cord, covered with the gelatin of Wharton 

 and the amnion, usually is about the size of the little finger. According 

 to Robin, the umbilical cord will sustain a weight of about twelve pounds 

 (5.4 kilos). As the amniotic liquid accumulates and distends the amnion, 

 this membrane becomes more and more closely applied to the cord. The 

 pressure extends from the placental attachment of the cord toward the 

 foetus, and it gradually forces into the abdomen of the foetus the loop 

 of intestine, which, in the early periods of intra-uterine life, forms an 

 umbilical hernia. 



The vessels of the cord the arteries as well as the vein are pro- 

 vided with valves. These are simple inversions of the walls of the vessels, 

 and they do not exist in pairs nor do they seem to influence the current 

 of blood. In the arteries these folds are situated at intervals of half an 

 inch to two inches (12.7 to 58.8 millimeters), and they are more abundant 

 where the vessels are very contorted. In the vein the folds are most 

 abundant near the placenta. They are irregularly placed, and in a 

 length of four inches (10 centimeters), fifteen folds were found (Berger). 

 It is not apparent that these valvular folds have any physiological 

 importance. 



As the allantois is developed, it presents, in the early stages of its 

 formation, three portions : an external portion, which becomes the 

 chorion, an internal portion, enclosed in the body of the embryo, and 

 an intermediate portion. The intermediate portion becomes the umbili- 

 cal cord. As the umbilicus of the foetus closes around the cord, it shuts 

 off a portion of the allantois contained in the abdominal cavity, which 

 becomes the urinary bladder; but there is a temporary communication 

 between the internal portion and the lower portion of the cord, called 

 the urachus. This usually is obliterated before birth and is reduced to 

 the condition of an impervious cord ; but it may persist during intra- 

 uterine life, in the form of a narrow canal extending from the bladder to 

 the umbilicus, which is closed soon after birth. 



At this time, when the ovum measures one-eighth to one-fourth of an 

 inch (3 to 6 millimeters) in diameter, the outer portion of the villi is 

 covered, over either a part of the ovum or the entire surface, with a 

 special layer of cells derived from the epiblast. These constitute the 



