DEVELOPMENT OF THE FOETUS. 41 



rating their own materials. The foetal portion of the placenta con- 

 sists of the branches of the umbilical vessels, which divide minutely 

 where they enter the organ, and constitute by their ramifications a 

 large portion of its substance, each subdivision terminating in a villus. 

 Each villus contains a capillary vessel, which forms a series of loops, 

 communicating with an artery on one side and with a vein on the 

 other. The vessels of the villi are covered by a layer of cells en- 

 closed in basement membrane. The maternal portion may be con- 

 sidered as a large sac, consisting of a prolongation of the internal 

 coat of the great uterine vessels. Against the foetal surface of this 

 sac the placental tufts push themselves, dipping down into it and 

 carrying before them a portion of its thin wall, so as to constitute a 

 sheath to each tuft. The blood is conveyed into the cavity of the 

 placenta by the " curling arteries,'^ so 

 named from their tortuous course, which Fig. 14.* 



proceed from the arteries of the uterus, and 

 the blood is returned through large uterine 

 veins called sinuses. The placenta per- 

 forms the twofold office of an absorbing 

 and respiratory organ ; it begins to be 

 formed about the end of the second month, 

 acquires its peculiar character during the 

 third, and goes on increasing in proportion 

 to the development of the ovum. At full 

 term its diameter is about six or eight inches, its circumference 

 eighteen to twenty-four, and its thickness from one inch to an inch 

 and a half. Its internal or foetal surface is smooth and shining, 

 being covered by the chorion and amnion ; the outer, or uterine sur- 

 face, is level but not so smooth, being divided by numerous sulci 

 between the lobules of which it is composed. 



Uie winbilical cord, called also the funis, or navel-string, is the 

 means of communication between the foetus and placenta. It usually 

 arises from the centre of the placenta, though sometimes from the 

 edge, forming the battledore placenta. It consists of two umbilical 

 arteries, and one umbilical vein ; besides these it contains the duct 

 of the umbilical vesicle, the omphalo-mesenteric vessels, the urachus, 

 and sometimes more or less of the intestinal canal, the whole im- 

 bedded in the Whartonian jelly, and invested by a reflection from the 

 amnion. The length varies much ; the average, however, is about 

 eighteen inches. Sometimes it is so short as seriously to impede the 

 progress of the labour. To return to the development of the embryo. 



* Extremity of a placental villus :— a, external membrane of the villus, continuous with 

 the lining membrane of the vascular system of the mother ; b, external cells of the villus, 

 belonging to the placental decidua ; c, c, germinal centres of the external cells ; d, the space 

 between the maternal and foetal portions of the villus; e, the internal membrane of the 

 villus, continuous with the external membrane of the chorion ;/, the internal cells of Uie 

 villus, belonging to the chorion ; g, the loop of umbilical vessels. 



4* 



