456 OF GENERATION AND DEVELOPMENT. 



supply of nutritious matter stored up in the ovum itself. The con- 

 tained embryo appropriates the fluid which is thus imbibed, by simple 

 absorption through its surface ; and thus it is nourished until a more 

 special provision for its development comes into action. The structure 

 of this organ, termed the Placenta, cannot be understood, until the 

 concurrent changes in the lining membrane of the Uterus have been 

 considered. 



810. This membrane, in its natural condition, presents on its free 

 surface the orifices of numerous cylindrical follicles ; which are arranged 

 parallel to each other, and at right angles to the surface. In the spaces 

 between these follicles, the blood-vessels form a dense capillary network. 

 When impregnation takes place, this mucous membrane swells and be- 

 comes lax ; its capillaries increase in size ; the follicles are developed 

 into glandular cavities, and become turgid with a white epithelium ; and 

 the interfollicular spaces are crowded with nucleated cells, which fill up 

 the meshes of the capillary network. In this peculiar condition, the 

 uterine mucous membrane is termed the Decidua. 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. Much discussion has taken place with regard to the 

 mode in which the decidua reflexa originates, and the question cannot 

 even now be considered as determined. The view recently put forth by 

 Coste is, perhaps, as probable as any. He considers that the ovum on 

 its entrance into the uterine cavity, is partly imbedded in its thick vas- 

 cular lining membrane, and that this swells up around it, like the 

 granulations around the pea in an issue ; so that at last the ovum be- 

 comes completely invested by the special envelope thus formed, which 

 closes in around it, constituting the decidua reflexa, and which is at 

 first not in contact with the decidua vera at any part save where it has 

 sprung from it. As the ovum increases in size, the decidua reflexa 

 grows with it, and is thus gradually brought into contact with the 

 decidua vera which lines the uterus, the cavity between them being 

 obliterated ; and at a later period, the two coalesce, so that they are no 

 longer distinguishable from each other. 



811. When the ovum has arrived in the Uterus, therefore, and the 

 villous tufts of its chorion are developed, these come into contact, in 

 the first instance, with the epithelial layer which intervenes between 

 them and the vascular decidua. Through this cellular membrane, 

 therefore, the ovum must derive its nutriment from the vascular sur- 

 face ; and it cannot be deemed improbable, that its office is to draw 

 from the subjacent vessels the materials which are to serve for the 

 nutrition of the ovum, and to present it to the villous tufts of the 

 chorion. Each of these, as already mentioned, is composed of an 

 assemblage of nucleated cells, which are found in various stages of 

 development ; and the villus seems to elongate by the development of 

 new cells from the germinal spot at its free extremity, whilst, like the 

 spongiole of the plant, it draws in nutriment from the soil in which it 

 is imbedded. On the other hand, the Decidua at this early period, 

 appears to be actively employed in preparing nutriment for the 

 embryo ; for its cellular layer is so abundant, as to form a bed into 



