708 A MANUAL OF VETERINARY PHYSIOLOGY 



through her system is indirectly brought into contact with the 

 blood in the vessels of the embryo, through the medium of a 

 vascular sac in which it is lying. The growth and development 

 of the coverings of the embryo must now be looked at. 



The Decidua. — At every monthly period in the human female the 

 mucous lining of the uterus undergoes certain changes, which 

 result in the formation of a membrane known as the decidua ; this 

 is in shape a counterpart of the interior of the uterus. The membrane 

 is shed during menstruation ; if the woman becomes pregnant the 

 decidua is not exfoliated, but undergoes further development in 

 connection with the ovum. The latter on its arrival in the uterus 

 becomes embedded in the folds of mucous membrane which grow 

 up around and anchor it to the wall of the uterus. That portion 

 of the mucous membrane which grows over and envelops the ovum 

 is known as the decidua reflexa, that which lines the interior of the 

 uterus is known as the decidua vera. Through the decidua vera the 

 uterine glands grow, and later on in embryonic life, when the final 

 circulation is established between the foetus and the mother by 

 means of the placenta, the latter on the maternal side is attached 

 to a portion of the decidua vera, and to this part the term decidua 

 serotina is given. After the birth of the child, the membrane 

 covering it, the placenta, and the uterine decidua, are all cast off, 

 with the result that the interior of the uterus is converted into a 

 large raw wound. 



Placenta. — No domesticated animal has a decidua ; the ovum is 

 attached in quite another way to the uterine wall, and, though a 

 placenta exists, it is differently arranged to that of the human 

 female. This has led to the primary classification of placentae into 

 deciduate and non-deciduate, but these terms, in the light of recent 

 inquiry, are not appropriate, for it is no longer a matter of importance 

 from a morphological point of view whether a portion of the maternal 

 tissue comes away with the afterbirth or not. Assheton* proposes 

 to group placentae into two great types, placenta cumulata and 

 placenta plicata, these terms being based on the arrangement of a 

 certain group of cells (the trophoblast) in the outer layer of the 

 embryo, through which the embryo is secured to the wall of the 

 uterus. Whatever form the placenta may be, or whatever the 

 attachment between the foetus and the mother, it is always originated 

 by the trophoblast cells. In the cumulate type of placenta the 

 trophoblast cells heap themselves up and destroy the uterine epithe- 

 lium, and form spaces into which the maternal blood escapes ; while 

 in the plicate there is no heaping up, but a process of folding and 

 ingrowth takes place, the uterine epithelium in most cases being left 

 intact. The pig is the extreme type of plicate placenta ; then follows 

 the mare, cow, sheep, while the placenta of man and carnivora is 

 of the cumulate type. It must not be supposed that these types 

 are sharply divided ; for instance, the sheep has a plicate placenta 

 which contains cumulate features, and the placenta of the dog, 

 though cumulate, has features of a plicate type. 



Besides recognising placentae as deciduate and non-deciduate, or 

 plicate and cumulate, they are further classified according to the 

 disposition of the chorionic villi. If the villi are scattered over the 



* ' On the Morphology of the Ungulate Placenta,' by R. Assheton, M.A., 

 Phil. Trans., B, vol. cxcviii., 1905. 



