46 CHANGES IN THE UTERUS. 



in birds, and in some mammals, e.g. ruminants, this portion forms a large sac which occupies 

 the greater part of the cavity of the false amnion, and is filled by fluid (allantoic fluid) in 

 which many urinary products can be recognized. (The ducts of the embryonic renal organs open 

 into the pedicle of the allantois.) But in most mammals the development of the hypoblastic 

 sac is far less extensive, and in some, including the human embryo, the allantois is mainly 

 represented by a large mesoblastic outgrowth carrying the allantoic (umbilical) vessels to the 

 chorion. There is also considerable variation in the period at which the allantois begins to 

 develop. In the human embryo it is certainly formed at a very early period, probably even 

 before the amnion is completed. In the guinea-pig, also, it appears early, although after the 

 amnion. In this animal it is first developed as a solid outgrowth of mesoblast which projects 

 from the line of junction of the hinder end of the amniotic bag with the blastoderm, and 

 before the formation of a hind gut or of any part of the alimentary tube, a hypoblastic 

 diverticulum being altogether wanting. In the earliest human ova in which the allantois 

 has been investigated, it is already a tube of hypoblast which forms a direct prolongation of 

 the posterior end of the primitive alimentary canal (fig. 49, d), and is enclosed in a short 

 stalk of mesoblast, by which the posterior end of the embryo is attached to the chorion, and 

 through which, by the allantoic (umbilical) blood-vessels, the chorionic villi are freely 

 supplied with blood. From the attachment of this stalk to the placenta (chorionic part), 

 the hinder extremity of the amnion is reflected. The stalk in question is not the um- 

 bilical cord, since it does not include the stalk of the yolk sac (vitelline duct), which only 

 later becomes bound up with it. It is termed by His the abdominal stalk (Bauchstiel), 

 the term allantois being by him restricted to the hypoblastic diverticulum. It is further 

 considered by His to be probable that the human embryo never becomes completely detached 

 from the chorion, but that it always retains its attachment to the outer membrane 

 of the ovum at the hinder end, this abdominal stalk being regarded as a direct prolongation 

 of the tail end of the embryo (fig. 49). If this should prove to be the case, the 

 human ovum would form an exception to the usual rule of a complete separation of embryo 

 from chorion at the formation of the amnion, and subsequent re-attachment by outgrowth of 

 allantois. 



Changes in the uterus. Mode of attachment of ovum to uterus. 1 



The mucous membrane of the pregnant uterus is known as the decidua. It is 

 thicker and more pulpy than in the ordinary non-pregnant condition, and the 

 glands are longer in proportion, but it is otherwise of similar structure except in 

 the part where the placenta is about to be formed ; here it undergoes important 

 modifications. The ovum, which has been fertilized and has passed through the 

 first stages of development in the Fallopian tube, although considerably larger than 

 the undeveloped ovum, is still an extremely minute object when it reaches the 

 uterus. Here it speedily becomes imbedded in the soft and thickened mucous 

 membrane, and this is soon reflected over and completely encloses the ovum, which 

 thus comes to lie in a cavity within the decidua which is altogether shut off by the 

 reflected part from the true cavity of the uterus. Different names have been given 

 to these parts of the uterine mucous membrane which immediately enclose the ovum 

 to distinguish them from that which lines the original cavity of the uterus. Thus 

 the layer of membrane which has grown around the ovum is known as the decidua 

 reflexa ; the part where the ovum first becomes attached to the uterus and where the 

 placenta is afterwards formed, is the decidua serotina, while the membrane lining 

 the true cavity of the uterus, is termed decidua vera (figs. 50, 51). 



With the subsequent growth and consequent expansion of the ovum the enclosing 

 decidua reflexa expands also pari passu, encroaching more and more upon the true 

 cavity of the uterus and corning into contact everywhere with the decidua vera. 

 Eventually it blends entirely with the decidua vera, so that the two layers are indis- 

 tinguishable and the original cavity of the uterus is obliterated (except at the 

 cervix uteri). 



1 The following account of the formation of the deciduse and of the placenta is confined as much as 

 possible to what has been observed in the human subject. In other mammals important variations in 

 the mode of attachment of the ovum and in the formation and structure of the placenta are found to 

 occur, 



