96 GENERAL ANATOMY. 



The umbilical cord appears about the end of the fifth month after pregnancy. 

 It consists of the coils of two arteries (umbilical) and a single vein, united to- 

 gether by a gelatinous mass (gelatin of Wharton) contained in the cells of an 

 areolar structure. There are originally two umbilical veins, but one of these 

 vessels becomes obliterated, as do also the two omphalo-mesenteric arteries and 

 veins, and the duct of the umbilical vesicle, all of which are originally con- 

 tained in the rudimentary cord. The permanent structures of the cord are 

 therefore those furnished by the allantois. 



Growth of the Embryo. Tbe youngest human embryos which have been met 

 with are two described by Dr. A. Thomson, in the "Edinb. Med. and Surgical 

 Journal, 1839," and in his paper references to the other extant descriptions of 

 early ova will be found. The ova in question were believed to be of the ages re- 

 spectively of twelve to fourteen days, and about fifteen days. 1 The figures are here 

 reproduced. The earliest ovum (Fig. 56) was fy of an inch in diameter, when 

 freed from some adherent decidua. The chorion presented a slightly villous ap- 



Fig. 56. Fig. 57. 



\v 



Human ovum, 12 to 14 dnys. Human ovum, Embryo from the preceding ovurn. 1. Umbilical 



1. Natural size. 2 Enlarged. 15 days. vesicle. 2. Medullary groove. 3. Cephalic por 



tion of the embryo. 4. Cnudnl portion. 5. Frag 

 inent of membrane (amnion?). 



; 



pearance, and consisted only of one layer of membrane. On opening it the um- 

 bilical vesicle and embryo were found not to fill its cavity completely. The 

 embryo was a line in length, and nearly ^ of an inch in thickness. The chorior. 

 was united to the embryo and umbilical vesicle by a thin tenacious web of albu- 

 minous filaments, formed probably by coagulation in the spirit in which it had 

 been kept. There were no vessels on the umbilical vesicle. The abdomen of the 

 embryo presented no appearance of intestine, but merely a long shallow groove, 

 forming a common cavity with the yelk-sac. Around this intestinal groove the 

 germinal membrane was continuous with that on the surface of the yelk-sac. 

 One extremity of the embryo, probably the cephalic, was enlarged, but this the 

 author believed to be accidental. A more opaque and expanded portion be- 

 tween the cephalic extremity and the surface of the yelk-sac appeared to him 

 to indicate the rudimentary heart. 



The second embryo (Figs. 57, 58) was in a slightly more advanced condition. 

 In it, as in the former, the amnion and allautois were not found, though the 

 adhesion of the embryo by its dorsal aspect to the inner side of the chorion 

 renders it probable that the amnion was fo.rmed. The cephalic and caudal 

 extremities could be easily distinguished; the vertebral groove appeared to be 

 open in its whole extent; there was a more perfect intestinal groove than in 

 the former case, and there was an irregular shaped mass between the yelk and 

 the cephalic extremity of the embryo, which Professor Thompson believed to 



1 For the data on which these calculations are founded, the reader is referred to the original 

 paper. 



