ACTION OF THE FEMALE. 703 



texture of Chorion seems to be produced, like the membranous basis of the 

 egg-shell of the Bird ( 118), by the exudation of Fibrine from the lining 

 membrane of the Fallopian tube or Oviduct. The outer layer of this en- 

 velope, in the egg of the Bird, is consolidated by the deposition of particles of 

 Carbonate of Lime in its areolrc ; but it undergoes no higher organization. 

 The Chorion of the Mammal, on the other hand, subsequently undergoes 

 changes of a much higher order; which adapt it for participating, to a most 

 important degree, in the nutrition of the included embryo. The first of these 

 changes consists in the extension of the surface of the membrane into a num- 

 ber of villous prolongations, which give it a spongy or shaggy appearance. 

 These serve as absorbing radicles, and form the channel through which the 

 embryo is nourished by the fluids of the parent, until a more perfect commu- 

 nication is formed, in the manner to be presently explained. 



919. We have now to speak of the changes in the Uterus, which take 

 place in consequence of Conception, and which prepare it to receive the 

 Ovum. Of these the most important is the formation of the Membrana De- 

 cidua, so called from its being cast off at each parturition. This membrane 

 has been usually supposed to be a new formation ; and has been described as 

 originating in coagulable lymph thrown out on the inner surface of the Uterus, 

 into which vessels are prolonged from the subjacent surface. It appears, 

 however, from the late researches of Dr. Sharpey and Prof. Weber,* that this 

 is not the true account of it; and that the Decidua is really composed of the 

 inner portion of the Mucous membrane itself, which undergoes a considerable 

 change in its character. The Mucous membrane of the Uterus had been ob- 

 served by Dr. J. Reid to possess, on its free surface, a tubular structure ; not 

 very unlike that which has been described as existing in the lining membrane 

 of the stomach ( 873 and Fig. 269). This tubular portion becomes thickened 

 and increased in vascularity, within a short time after conception ; and when 

 the inner surface of a newly-impregnated Uterus is examined with a low 

 magnifying power, the orifices of its tubes (Plate I., Fig. 17, b, b) are very 

 distinctly seen, being lined with a white epithelium. The blood-vessels (c, c) 

 form a very minute net-work, which extends in loops from the subjacent 

 portion of the membrane. According to the recent observations of Mr. J. 

 Goodsir,t the interfollicular spaces (a, a, a) also are crowded with nucleated 

 particles ; and it is to the development of this interfollicular substance, as well 

 as to the enlargement of the follicles themselves, and the copious development 

 of epithelial cells in their interior, that the mucous membrane in this condition 

 owes its increased thickness. At a later period, the Decidua may be found 

 to consist of two distinct layers; the Decidua vera, lining the uterus; and the 

 Decidua reflexa, covering the exterior of the ovum. It was formerly supposed 

 that the latter is a portion of the former, which has been pushed before the 

 ovum at its entrance into the uterus ; but the two layers are so different in 

 texture, that they cannot be supposed to have the same origin. The difficulty 

 appears to be solved by the observations of Mr. Goodsir. " From what has 

 now been stated," he remarks, " it appears that the Decidua consists of two 

 distinct elements; the mucous membrane of the uterus, thickened by a pecu- 

 liar development ; and a non-vascular cellular substance, the product of the 

 uterine follicles. The former constitutes, at a later period, the greater part of 

 the decidua vera; the latter, the decidua reflexa. This view of the consti- 

 tution of the Decidua clears up the doubts which were entertained regarding 

 the arrangement of these membranes at the os uteri and entrances of the Fallo- 

 pian tubes. It is evident that these orifices will be open or closed, just as the 



* Miiller's Physiology, pp. 1574-1580. 



f Anatomical and Pathological Observations, Chap. ix. 



