FCETAL MEMBRANES. 



73 



somewhat different. The first part or its origin forms the urinary bladder, and 

 from the vertex of the latter there proceeds through the umbilicus a tube, the 

 urachus, which is open at first (fig. 665, VIII, a). The blind part of the sac of 

 the allantois outside the abdomen is in some animals filled with a fluid like urine. 

 In man, however, this sac disappears during the 2nd month, so that there remains 

 only the vessels which lie in the muscular part of the allantois. In some animals, 

 however, the allantois grows larger, does not shrivel, but obtains through the 

 urachus from the bladder an alkaline turbid fluid, which contains some albumin, 

 sugar, urea, and allantoin. The relations of the umbilical vessels will be described 

 in connection with the foetal membranes. 



445. FCETAL MEMBRANES, PLACENTA, FOETAL CIRCULATION. 



Decidua. When a fecundated ovum reaches the uterus, it becomes surrounded by 

 a special covering, which William Hunter (1775) described as the membrana 

 decidua, because it was shed at birth. We distinguish the decidua vera (fig. 665, 

 VIII, p), which is merely the thickened, very vascular, softened, more spongy, and 

 somewhat altered mucous membrane of the uterus. [Sometimes in a diseased con- 

 dition, as in dysmenorrhea, the superficial layer of the uterine mucous membrane is 

 thrown off nearly en masse in a 

 triangular form (fig. 668). This 

 serves to show the shape of the 

 decidua, which is that of the 

 uterus.] When the ovum 

 reaches the uterus, it is caught 

 in a crypt or fold of the de- 

 cidua, and from the latter there 

 grow up elevations around the 

 ovum; but these elevations are 

 thin, and soon meet over the 

 back of the ovum to form the 

 decidua reflexa (fig. 665, 

 VIII, r). At the 2nd to 3rd 

 month, there is still a space in 

 the uterus outside the reflexa ; 

 in the 4th month, the whole 

 cavity is filled by the ovum. 

 At one part the ovum lies 

 directly upon the d. vera [and ri &- 668 - 



that part is spoken of as the A dysmenorrhceal membrane laid open. 



decidua serotina], but by far the greatest part of the surface of the ovum is in 

 contact with the reflexa. In the region of the d. serotina the placenta is ulti- 

 mately formed. 



Structure of the Decidua Vera. The d. vera at the 3rd month is 4 to 7 mm. thick, and 

 at the 4th only 1 to 3 mm. , and it no longer has any epithelium ; but it is very vascular, and 

 is possessed of lymphatics around the glands and blood-vessels {Leopold), and in its loose sub- 

 stance are large round cells (decidua cells Kolliker), which in the deeper parts become changed 

 into fibre cells there are also lymphoid cells. The uterine glands, which become greatly 

 developed at the commencement of pregnancy, at the 3rd to the 4th month form non-cellular, 

 wide, bulging tubes, which become indistinct in the later months, and in which the epithelium 

 disappears more and more. The d. reflexa, much thinner than the vera from the middle of 

 pregnancy, is devoid of epithelium, and is without vessels and glands. Towards the end of 

 pregnancy both deciduse unite. 



The ovum, covered at first with small hollow villi, is surrounded by the decidua. 

 From the formation of the amnion it follows that, after it is closed, a completely 

 closed sac passes away from the embryo to lie next the primitive chorion. This 

 membrane is the "serous covering" of v. Baer (fig. 665, VII, s), or the false 



