704 OF REPRODUCTION. 



cellular secretion is more or less plentiful, or in a state of more or less vigor- 

 ous development." " When the ovum enters the cavity of the uterus, the 

 cellular decidua surrounds it, and becomes what has been named the decidua 

 reflexa, by a continuation of the same action, by which it had been increasing 

 in quantity before the arrival of the ovum. The cellular decidua grows around 

 the ovum by the formation of new cells, the product of those in whose vicinity 

 the ovum happens to be situated." 



920. When the Ovum has arrived in the Uterus, therefore, and the villous 

 tufts of the Chorion are developed, these come into contact, in the first in- 

 stance, with the layer of cellular decidua, which intervenes between them 

 and the vascular decidua. Through this cellular membrane, therefore, the 

 ovum must derive its nutriment from the vascular surface; and it cannot be 

 deemed improbable, that the office of its component cells 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 is 

 composed (according to the observations of Mr. J. Goodsir) of an assemblage 

 of nucleated cells, which are found in various stages of development ; and 

 these are always inclosed within a layer of basement-membrane, which 

 seems to be itself composed of flattened cells united by their edges. At the 

 free extremity of each villus, is a bulbous expansion, the cells composing 

 which are arranged round a central spot ; and it is at this point that the most 

 active processes of growth take place, the villus elongating by the develop- 

 ment of new cells from its germinal spot, and (like the spongiole of the plant) 

 drawing in. nutriment from the soil in which it is imbedded. In its earliest 

 grade of development, the chorion and its villi contain no vessels ; and the 

 fluid drawn in by the tufts is communicated to the embryo, by the absorbing 

 powers of the germinal membrane of the latter. But when the tufts are pe- 

 netrated by blood-vessels; and their communication with the embryo becomes 

 more direct, the means by which they communicate with the parent are found 

 to be still essentially the same ; namely, a double layer of nucleated cells, 

 one layer belonging to the foetal tuft, and the other to the vascular maternal 

 surface. It is from these elements that the Placenta is formed, in the manner 

 next to be described. 



921. The first stage in this process consists in the extension of the foetal 

 vessels into the villi of the Chorion over its entire surface, in the manner here- 

 after to be detailed ( 941) ; so that the nutriment which these villi imbibe, 

 instead of being merely added to the albuminous fluid surrounding the yolk- 

 bag, is now conveyed directly to the embryo. This, the earliest and simplest 

 mode by which the Foetus effects a new connection with the parent, is the 

 only one in which it ever takes place in the lower Mammalia, which are hence 

 properly designated as " non-placental," rather than as ovo-viviparous ( 44). 

 In the higher Mammalia, however, there soon occurs a great extension of the 

 vascular tufts of the foetal Chorion, at certain points ; and a corresponding 

 adaptation, on the part of the Uterine structure, to afford them an increased 

 supply of nutritious fluid. These specially-prolonged portions are scattered, 

 in the Ruminantia and some other Mammalia, over the whole surface of the 

 Chorion, forming what are termed the Cotyledons ; but in the higher orders, 

 and in Man, they are concentrated in one spot, forming the Placenta. In 

 some of the lower tribes, the maternal and the foetal portions of the Placenta 

 may be very easily separated ; the former consisting of the thickened Decidua; 

 and the latter being composed of the prolonged and ramifying vascular tufts 

 of the Chorion, dipping down into it. But in the Human Placenta, the two 

 elements are mingled together through its whole substance. 



922. On looking at the Postal surface of the Human Placenta, we perceive 

 that the umbilical vessels diverge in every direction from the point at which 



