720 



UTERUS AND ITS APPENDAGES. 



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 author last 

 quoted, blood-vessels first appear in these parts 

 when 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 the 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 oeco- 

 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 in fig. 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. 



Such are the principal modifications which 

 normally take place during the development 

 and growth of the foetal portion of the pla- 

 centa. The changes occurring in the maternal 

 portion, or that which is supplied by the de- 

 cidua, are not less remarkable. 



Maternal portion. Four principal stages 

 may be observed in the formation of this por- 

 tion of the placenta. 



The first stage is that in which the decidua 

 constitutes a perfectly spherical chamber *j* 

 surrounding the ovum, but having as yet no 

 structural connection with it (fig. 486.). This 

 is the condition of the ovum in the early part 

 of the first month of gestation. 



The second stage is marked by the com- 

 mencing attachment of the villi all round to 

 the inner surface of the containing chamber, 

 so that now the ovum becomes fixed, and can 

 no longer be turned out, except by breaking 

 off the villi, or drawing out their ends from 

 the little pits, or anfractuosities, already de- 

 scribed in the walls of the decidua, in which 

 they have become embedded. At this period 

 (latter half of the first month), the decidua 

 forming the walls of this chamber is suffi- 

 ciently firm to admit of dissection, and already 

 there may be traced, upon its inner surface, 



* Loc. cit. pi. i.fig. 1. 



f For an account of the formation of the foetal 

 chamber, and of the early steps in the construction 

 of that portion of the placenta which belongs to the 

 decidua, see p. 653. 



Fig. 486. 



Decidua at the beginning of gestation, exhibiting the 

 foetal chamber in the first stage of its formation. 

 T/ie ovum, being at this time unattached, has dropped 

 out of it. (After W. Hunter.} 



orifices communicating with canals in the de- 

 cidua that lead into the uterine sinuses. The 

 maternal blood already flows freely into the 

 foetal chamber, and, after passing everywhere 

 among the villi, is returned into the uterine 

 veins. Thus a temporary placenta is formed, 

 which, as in Pachydermata, Cetacea, &c., en- 

 tirely surrounds the ovum (fig. 487.). 



The third stage is the most important. It 

 marks the transition from the temporary to 

 the permanent form of the placenta. Coinci- 

 dently with the increased development of the 

 villi on one side of the chorion, and their cor- 

 responding arrest of growth on the opposite 

 surface, there occurs an increase of the space 

 between the decidua and the ovum on one 

 side, and a corresponding decrease of it upon 

 the opposite side. The increase is always on 

 the side next the uterus, where the villi are 

 most abundant, and the decrease upon the 

 opposite surface, where they are fewest. And 

 this change continues progressively, until, 

 upon the bald side of the ovum, the decidua 

 reflexa and the chorion come into so close 

 contact that the interspace is obliterated, and 

 the blood, which formerly flowed freely among 

 the villi, is now no longer admitted to this 

 part of the circumference of the ovum ; while, 

 upon the side which is directed towards the 

 uterus, a large space is left which now takes 

 the form of a meniscus. In order more effec- 

 tually to confine the blood within this limited 

 space, an increased development of decidual 

 cells now takes place, which pass off from the 

 uterine walls, and attach themselves to the 

 chorion all round the circumference of this 

 space, and thus is formed the margin of the 

 permanent placenta. During all this time, the 

 ovum, by its growth, has been gradually rais- 

 ing the decidua above and around it, just as 

 the common integument becomes raised dur- 



