Termination of the maternal No extension of the maternal blood-vessels into the substance of the placenta among or be tween the villi, can be demonstrated to take place. So far as anatomical evidence goes, the maternal vessels all terminate at once and abruptly upon the inner surface of the clecidua. The curling arteries, after passing from the muscular coat of the uterus, obliquely for the most part, through the layer of decidua which forms the roof of the placenta, open directly into the interior of the latter ; while the veins commence by equally abrupt openings which conduct through the decidual layer to the venous sinuses in the uterine walls. These venous orifices occupy three situations. The first and most numerous are scattered over the inner side of the general layer of decidua which constitutes the upper boundary of the placenta ; the second form openings upon the sides of the decidual prolongations or dissepi ments, which separate the lobes from each other ; while the third lead directly into the interrupted channel in the margin, termed the circular sinus.
Development of the The early steps in the formation of the placenta have been described in the account which has been already given of the development of the deci dua during gestation (p. 653.). These first steps consist in the formation out of the deci dua of a perfectly spherical chamber, in the centre of which lies the inipregnated ovum.
The surface of the ovum is at this time covered every-where by short club-like villi of equal size. The extremities of these villi are simply in contact with, but are not as yet attached to the walls of the containing cham ber. Subsequently both the villi and the de cidua forming the fcetal chamber undergo con siderable metamorphoses. Certain portions of these become intimately united, in order to form the placenta ; while other portions suffer retrogression, and take no part in its construc tion. The following are the principal features in these metamorphoses.
Fatal The surface of the ovum does not long retain the peculiarity just men tioned, of being equally covered by villi. Dur ing the second month at least, if not earlier, those villi on the side furthest from the uterus cease to grow, and in consequence of the increasing expansion of the ovum become more widely scattered over this part of its surface, while those nearest to the uterus rapidly increase in size and extent, so that this portion of the ovum soon exhibits a pro fuse growth of villous processes, which send out their ramifications in all directions.
According to Professor Goodsir, the deve lopment and growth of the villi proceed from the groups of cells already described as occu pying their bulbous extrernities. These swell
ings on the sides and ends of the villi are their germinal spots, and are the active agents in the formation of these parts. The villus elongates by the addition of cells to its extre mity, the cells passing off from the gerrninal spot, and the spot receding on the extremity of the villus, as the latter elongates by the additions which it receives from it.
As the villi increase in size, their strength is gradually augmented by the conversion of the membrane and cells forming their stems and larger branches into a tough white fibrous texture ; while frequently, towards the end of gestation, calcification is observed to begin mithin the finer villi, and to proceed sometiines to so great an extent that a considerable num ber of them become filled up and obliterated by solid matter. While these changes are going on in the outer portion of the villi, or that which is derived from the chorion, im portant modifications occur in the interior structures. Up to a certain period of gesta tion, the chorion and its villi contain no blood-vessels. According to the anchor last quoted, blood-vessels first appear in these parts vvhen the allantois reaches and applies itself to a certain portion of the interior surface of the chorion. The umbilical vessels then commu nicate with the substance of the villi, and be come continuous with loops in their interior. Those villi in which tbe blood-vessels do not undergo any further development, as the ovum increases in size, become more widely sepa rated, and lose their importance in the ceco nomy. The villi, again, in which vessels form, in connection with the umbilical vessels, in crease in number, and undergo certain changes in the arrangement of their constituent ele ments. As the blood-vessels increase in size, the cells diminish in number, but are always found surrounding the terminal loop of ves sels in the situation of the germinal spot.
The injections of Schrceder van der Kolk* show a profusion of capillaries within the villi as early as the third month. And at later periods of gestation, up to the sixth month, I have succeeded without difficulty in display ing, by the aid of fine injections, such an abundant development of these vessels, as is exhibited infig. 485. Before the end of ges tation, however, the greater part or all of these fine capillaries have disappeared, and the vessels within the villi then show only the long tortuous varicose loops which Good sir has so well described.