Illi: H.'.TM. Ml M \MM VL8. 





i-nal hlimd-pass-iire.s is hill . for the formation of 



tin' placenta ill the 1 >eciduat a a- upp >! to that of the I ndecidnata. 



The second fin in, tl, -t. is characteri.-' ic of the 



Kodentia, the I nseetivora. t he Chiroptera and lYoMinise, the Apes and 

 Man. Here the portion of the chorion devoted to the formation of 

 the placenta is small ; but in compel .-at ion i'or this the tufts of villi 

 (tiir. 13s F) are' "very highly developed ; the union between placenta 

 uterina( .I/) and placenta 

 fn-talis ( /') is most 111- 

 timate ; the maternal 

 Hood spaces (d')j in the 

 (a- of the Apes and 

 Man at least, are. I 

 where else, enormously 

 distended, so that the 

 villi of the chorion (F) 

 appear to sink directly 

 into them and to be 

 -d immediately by 

 the maternal Mood. 



Since wo shall occupy 

 ourselves more at length 

 in the next chapter 

 with the human pla- 

 centa, which belongs to 

 tli is type, these few 

 remarks may suffice for 

 the time being. 



I elo>e this section 

 \\ith a reference to the 

 high systematic signifi- 

 cance of the embryonic 

 -ry organs of Ver 

 tebi-ates. They present, 

 as we have seen, sm h 

 ureat and >trikin^ dif- 

 ferences in the separate 



fig. 138. Diagrammatic representation of the finer struc- 

 ture of the human placenta according to the hypothesis 



liXKK. 



F, Foetal, J/", n au-rn.il j j>ithelimn of the 



maternal n:.il blood- 



vessels; V, villi; ua serotina of the human 



l/act-Mta ; t. t, tralx-culae of the sero.ina running to 

 tin- t'i-ial villi ; ra, convoluted artery which sinks into 

 the blood-space d' ; up, one of the utero-placental veins 

 conveying blood from the latter; x, a continuation 

 r the villus of maternal tissue lying outeide the 

 r|,i; .h it-present* either the endo- 



11 in <>f tin- <K!- vessels or a delicate 



m:.! t.i the serotina, or both 



together. The layer >' consist*, at all events, of ma- 

 :-iri the serotina. The foetal 



< pUhelial !a\er is n<> luiiL-er to be seen on the villi of 

 ..nijiletely fmmcd human placenta. 



that the utilisa- 

 tion of them for systematic purpo.-.-s \\hich has l>een made by 

 .MII.NK. KDWAUIJS, <>\\i\. and HUXLEY was natural. 



All lower Vertebrates. Amphioxns, ( 'yclostomes. Kishes. Dipnoi 



