260 EMBRYOLOGY. 



membrane, the memhrana chorii (m), in which the chief branches 

 of the umbilical arteries and veins take their course. They consist 

 of (1) large main stems («), which grow straight out from the mem- 

 brana chorii, and the ends of which {h}) sink into and firmly unite 

 with the placenta uterina, which faces them, and (2) numerous lateral 

 branches {/) which arise on all sides at right angles or obliquely, 

 and which are in turn covered with fine twigs. A small part of 

 these (h^) also fuse, by means of their tips, with the tissue of the 

 placenta uterina (Langhans), so that a separation of the foetal and 

 the maternal portions can be accomplished only by forcible detach- 

 ment. KoLLiKER has therefore appropriately divided the branches 

 of the chorionic villi into roots of attachment {h^, h^) and free 'pro- 

 cesses {/). 



To each arborescent chorionic villus there goes a large branch of 

 an umbilical artery, which, corresponding to the ramifications of the 

 former, is divided up into branches ; the capillary networks which 

 arise from this are situated quite superficially immediately under 

 the epithelium of the vUli. From this network the blood is collected 

 into vessels, leading from the villi, which are again united into a 

 single chief stem that emerges from the chorionic tuft. 



Consequently the vascular system of the placenta foetalis is entirely 

 closed. A direct mingling of the foetal and maternal blood cannot 

 take place in any manner ; on the other hand the prerequisite for 

 an easy exchange of fluid and gaseous components of the blood 

 is furnished by the very superficial position of the thin-walled 

 capillaries. 



Plate II. 



Diagrammatic section through the human 2>laBenta at the middle of tlie fifth 

 month, after LEOPOLD. 



The musculature of the uterus is followed by the spongy layer of the decidua 

 serotina {sp), in which the separation of the placenta takes place at birth 

 along the line of separation indicated by two heavy marks ; this is followed 

 by the compact layer ( CS), which is thrown oil at birth as the placenta uterina, 

 and which consists of the (Winkler's) basal plate {BP), closing plate (Schluss- 

 platte) i_SP), cavernous blood-spaces (o), the arteria advehentes (a), and. the 

 marginal sinus. The placenta foetalis has grown into the placenta uterina; 

 it consists of the membrana chorii (m) and the villi (z) arising from it ; on 

 the latter are to be distinguished the roots of attachment (A', K') and the free 

 processes (/). [ep, Koetal epithelium derived from the serosa.] The chorion 

 is still covered internally by the amnion. [The foetal part of the placenta is' 

 reproduced in blue, the maternal part in black and brown ; pink indicates the 

 blood-spaces.] 



