DECIDUA 67 



analogy is to be found in those mammals in which there is so-called ' inversion of 

 the germinal layers,' the Muridce (mice and rats) and Cavia (guinea-pig) among 

 the rodents. In some way the early nesting of the ovum in the decidua is related 

 to the inversion of the layers, and both phenomena are probably to be correlated 

 with the very minute size of the blastocyst. 



The idea that the imbedding occurs by ' circumvallation ' being given up, 

 there remain two possibilities either (a) that the minute ovum is received into 

 a crypt of the mucous membrane, or (b) that it, in virtue of a biochemical action 

 of the trophoblastic ectoderm, absorbs the epithelium, and eats its way into the 

 connective tissue of the mucous membrane. If the first alternative be adopted, 

 we must conceive the process to take place as it does in the mouse. 1 In this 

 animal the small blastocyst is received into a recess of the uterine cavity. The 

 epithelium lining this cavity becomes flattened and then disappears, so that 

 the trophoblast and decidua become closely related. The ectoplacenta blocks 

 the aperture between the decidual cavity and the lumen of the uterus, and the 

 cavity later becomes obliterated at the site of implantation, by the disappear- 

 ance of its epithelium and fusion of the exposed decidual walls. The gland-tubes 

 disappear as the mucous membrane becomes converted into decidua, and new 

 capillaries are freely produced, especially in the neighbourhood of the ectoplacenta, 

 where the maternal part of the placenta is formed. 2 



If the second alternative be adopted, then we have to conceive the process as 

 described by Graf v. Spee 3 for the guinea-pig. In that case the ovum reaches 

 the uterus in the morula or early blastocyst stage, and destroys the epithelium 

 at the point of contact with it. It becomes imbedded by a process of degeneration 

 of the connective tissue, which ultimately forms a sort of granulation-tissue capsule 

 around it. 



The evidence afforded by Peters', Leopold's, and other early ova is strongly in 

 favour of the view that the ovum actually absorbs the uterine tissue before it, 

 and therefore becomes implanted by the absorptive activities of its ectodermic 

 covering. In either case, at a very early stage the blastocyst lies surrounded on 

 all sides by mucous membrane without any trace of an epithelial layer. 



Chang-es in the uterus during* pregnancy. The mucous membrane of 

 the pregnant uterus is known as the decidua. For convenience of description, the 

 parts of the mucous membrane immediately enclosing the ovum, and that lining 

 the general cavity of the uterus, have received distinctive names. Thus the layer 

 of membrane around the ovum is known as the decidua capsularis (reflexa) ; the part 

 next the uterine wall where the placenta is afterwards formed is the decidua basalis 

 (serotina) ; while the membrane lining the cavity of the uterus is termed decidua 

 vera (fig. 94). 



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

 decidua capsularis expands also pari passu, encroaching more and more upon 

 the cavity of the uterus and coming into contact everywhere with the decidua 

 vera. Eventually it blends entirely with the decidua vera, so that the two layers 

 are indistinguishable and the cavity of the uterus is obliterated (except at the 

 cervix uteri). 



The ovum is received into the uterus when the mucous membrane is in the 

 premenstrual phase, and in the earliest pregnancies described (Peters' and Leopold's 

 cases) the mucosa has all the characters of the menstrual decidua (fig. 90). It is 



1 See G. Burckhard, Archiv mikr. Anat. Ivii. 



2 Disse (Sitzungsber. Ges. Beford. gesatnmt. Naturwiss., Marburg 1005) has shown that in mice and 

 rats there occur large giant-cells in the developing decidua, which have a phagocytic action, and 

 excavate it for the growing ovum. See also Disse, Ergebnisse dor Anat. und Entwick. xv. 1905. 



5 Zeitschr. Anat. u. Anthropol. iii. 



