1162 EMBRYOLOGY. 



the decidua of the uterus and probably absorb from it nutritive materials for 

 the growth of the embryo : they can be forcibly withdrawn from the decidua 

 until the third month of pregnancy. Until about the end of the second month 

 the villi cover the whole surface of the chorion and are of an almost uniform 

 size, but after this they develop unequally. On that part which invades the 

 decidua serotina they increase greatly in size and complexity, and constitute the 

 chorion frondosum, which becomes the foetal part of the placenta (Fig. 708). 

 Over the remainder of the chorion they undergo atrophy, so that by the fourth 

 month hardly any trace of them is left, and hence this part becomes smooth, 

 and is therefore named the chorion Iceve. The chorionic villi are at first non- 

 vascular, but subsequently they become vascularized by the growth into them 

 of the allantoic mesoblast, which carries to them the branches of the allantoic 

 arteries. 



The Allantois. The allantois grows outward as a hollow bud from the hind 

 gut, and is therefore lined by hypoblast and covered by mesoblast (Figs. 702, 4, 5, 

 and 703). It is projected into the space between the amnion and the chorion, 

 and in its mesoblast are carried a pair of arteries, the allantoic or umbilical arteries, 

 which are continued from the twO primary aortge. The allantoic mesoblast 

 gradually spreads out on the inner surface of the chorion, and, invading the 

 chorionic villi, supplies them with blood-vessels. In this way the allantois 

 becomes the chief agent of the foetal circulation, since it carries the vessels which 

 convey the blood of the embryo to the chorion, where it is exposed to the influence 

 of the maternal blood circulating in the decidua ; from the maternal blood it 

 imbibes the materials of nutrition and to it it gives up effete materials, the removal 

 of which is necessary for the purification of the foetal blood. In some animals the 

 allantois is a hollow projection, and is usually styled the allantoic vesicle ; but in 

 most mammals, and especially in man, the external or mesoblastic element under- 

 goes great development, while the internal or hypoblastic element undergoes little 

 increase beyond the body of the embryo, so that it is very doubtful whether any 

 cavity exists in the allantois beyond the limits of the umbilicus, or whether it does 

 not rather consist of a solid mass of material derived from the mesoblastic tissue. 1 

 The proximal part of the allantoic vesicle within the body-cavity is eventually 

 destined to form the bladder, while the remainder forms an impervious cord, the 

 urachus, stretching from the summit of the bladder to the umbilicus. The part 

 of the allantois external to the foetus forms the umbilical cord, by which the 

 foetus is connected with the placenta. 



The Decidua. The growth of the chorion and placenta can be understood only 

 by tracing the formation of the decidua. 



The decidua is formed from the uterine mucous membrane before the fertilized 

 ovum reaches the cavity of the uterus. The mucous membrane becomes vascular 

 and tumid, its glands are greatly elongated, and their deeper portions are dilated 

 and tortuous, while the interglandular tissue becomes crowded with epithelial- 

 like cells (decidual cells}. The mucous membrane, thus altered, is named the 

 decidua vera ; it lines the cavity of the uterus as far as the os internum, without, 

 however, occluding the orifices of the Fallopian tubes. When the fertilized ovum 

 reaches the uterus, which is thus prepared for its reception, it becomes attached 

 to the decidua, in most cases in the neighborhood of the lundus uteri. The decidua 

 then grows up around the ovum and ultimately covers it in. The part of the 

 decidua which grows up to envelop the ovum is named the decidua reflexa ; that 

 portion to which the ovum originally became attached is termed the decidua 

 serotina, and from it the maternal part of the placenta is derived. After concep- 

 tion the cervix uteri is closed by a plug of mucus (Fig. 708). 



By the fourth month the decidua vera has acquired a thickness of about half 



1 Indeed, it would appear, from the researches of His, that in the human embryo the allantois is 

 formed unusually early, being present from a very early period as a stalk of mesoblast connecting the 

 posterior extremity of the embryo with the chorion. This stalk is termed the abdominal stalk 

 (Bauchstiel). 



