PLACENTA (DEVELOPMENT). 



719 



explain the exact relations of this structure to 

 the villi, within the placenta. All the extre- 

 mities of the villi which are sufficiently long 

 to reach across the placenta from the chorion 

 to the opposite surface formed by the decidua, 

 become firmly attached to the inner side of 

 the latter. This attachment takes place not 

 by any actual perforation of the decidua, hut 

 by the ends of the villi being simply inserted, 

 in an early stage of the formation of the pla- 

 centa, into little shallow pits or cup-like de- 

 pressions in the decidual substance, into which 

 they are received, and from which they may 

 be withdrawn.* In other cases, the ends of 

 the villi become blended with the decidua, to 

 which they are apparently fixed, by a growth 

 of decidual cells. These attachments are for 

 the purpose of giving strength to the placenta, 

 and of mechanically supporting the villi. They 

 take place not only between the ends of the 

 villi and the decidua forming the roof of the 

 placenta, but also wherever decidua and villi 

 come into contact. Hence similar attachments 

 are also formed between the villi and the septa 

 or dissepiments (Jig. 484. d p), which divide 

 its substance into separate lobes. Upon t he floor 

 also of the placenta all round the margin, where 

 the decidua turns downwards and inwards to 

 become united with the chorion, and to form 

 the placental margin, the decidua is found for 

 a short distance attached to the bases of the 

 villi. And this arrangement gives to the parts 

 an appearance as if the decidua had been here 

 penetrated by the villi, but one which is actu- 

 ally occasioned by the former having, in the 

 course of growth, become extended around 

 the roots of the latter long after these were 

 first formed. Occasionally also decidual cells 

 may be found upon the surface of villi, con- 

 necting together their extremities, or forming 

 here and there rough irregular belts upon their 

 stems. 



Termination of the maternal vessels. 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 decidua. 

 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 



* The difficulty of understanding the early steps 

 in the construction of the placenta has arisen from 

 the belief commonly prevalent, that the ovum on 

 first reaching the uterus remains upon the outside 

 of the decidua, and that the villi of the chorion 

 penetrate its substance or enter the uterine glands 

 in order to form the placenta. But there is no 

 actual penetration of the decidua at any period, 

 except that which consists in the entire ovum 

 gaining a situation in the interior of this membrane 

 shortly after its arrival in the uterus. The tips of 

 the villi at a certain stage, as above described, be- 

 come superficially imbedded in the walls of the 

 fcetal chamber, which is formed of decidua ; but 

 this is not a penetration of the decidua, as commonly 

 understood, but only a means of fixing the ovum. " 



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 placenta. 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 impregnated ovum. 



The surface of the ovum is at this time 

 covered everywhere 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 foetal 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. 



Foetal portion. 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 extremities. 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 germinal 

 spot, and the spot receding on the extremity 

 of the villus, as the latter elongates bj 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 

 within the finer villi, and to proceed sometimes 

 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 



