716 UTEROGESTATION. 



that when it subsequently enlarges it carries with it, or there is formed 

 round it a covering of the membrane, the substance of which is at 

 the same time undergoing a rapid decidual development, and that this 

 substance continuing to grow with the ovum and expanding with it, 

 constitutes the decidua reflexa. The entire similarity of the structure 

 of the decidua reflexa at its base with that of the decidua vera is 

 in favour of the view that it owes its origin to a similar mode of 

 production. 



The formation of the decidua is, as has already been stated, to be 

 attributed mainly to a great increase in the development of the sub- 

 epithelial tissue. Its substance, accordingly, consists in great measure 

 of the cells, round and spindle-shaped, and cell-fibres which belong 

 to that tissue ; but these are mingled with much larger irregularly- 

 formed multi-nuclear cells, which increase in number as pregnancy 

 advances, and which are peculiarly characteristic of the structm'e of 

 the outer layers of the decidua. 



The blood-vessels and the glands of the mucous membrane also 

 undergo great enlargement and modification. The whole of the decidua 

 vera and the basilar part of the reflexa are at first penetrated by 

 blood-vessels derived from those of the uterus, more especially in the 

 latter part of the second and first, half of the third months, Avhen the 

 decidual structure may be considered as having reached its highest 

 degree of development. After this time the blood-vessels of the 

 decidua reflexa, and later those of the whole lining decidua of the 

 uterus, except in the immediate vicinity of the placenta, shrink and 

 ultimately disappear, so that the united decidua becomes in the end 

 wholly non-vascular. The same retrograde process, or atrophy and 

 disappearance, occurs in the blood-vessels of the chorionic villi by which 

 the decidua reflexa is penetrated, and, although the villi themselves 

 never entirely disappear, but may be traced even in the advanced stages 

 of pregnancy as sparse and shrivelled irregular arborescent processes, 

 the blood-vessels very soon begin to shrink and disappear from all the 

 villi ^hich do not form part of the placental structure. 



The uterine glands also become enlarged during the development of 

 the decidua, being both elongated in their deeper convoluted portions, 

 which are directed towards the muscular wall of the uterus, and under- 

 going a peculiar change not yet fully understood, in the parts next 

 their openings on the inner surface. Over the surface of the whole 

 decidua vera, as it lines the uterine cavity, and also on the decidua 

 reflexa, except at its most projecting part, a number of irregular pits 

 are visible to the naked eye, which are frequently so numerous as to 

 give the membrane a reticulated or sieve-like appearance (cups of 

 Montgomery). These pits are really the uterine glands enlarged and 

 altered soon after the commencement of pregnancy, as first clearly 

 shown by Sharpey (Midler's Physiology by Baly, 1842, p. 1579), and 

 the fact has since been observed by others (Kolliker, Coste). The 

 villi embedded in the decidua do not, however, occupy the cavities of 

 these pits, but so far as yet ascertained, are rather sunk in the inter- 

 glandular hypertro])hie*d substance of the decidua between them 

 (Schroder van der Kolk, Kolliker, and Priestley. See the lectures by 

 the latter " On the Development of the Gravid Uterus," 1860, p. 24). 

 Upon the more exact relation of the villi to the uterine glands in the 

 placenta, further remarks will be made hereafter. 



