«“ 
REPRODUCTION. 81 
allantois small but vascular; the former is said (Owen) to be 
attached to the subzonal membrane, the latter not; but no villi, 
and consequently no true chorion, is developed. All mammals 
other than the monotremes and marsupials have a true allan- 
toic placenta. : 
The Discoidal Placenta.—This form of placenta is that existing 
in the rodentia, insectivora, and cheiroptera. The condition 
found in the rabbit is that which has been most studied. The 
relation of parts is shown in Fig. 83. 
The uterus of the rodent is two-horned; so we find in gen- 
eral several embryos in each horn in the pregnant rabbit. They 
are functionally independent, each having its own set of mem- 
branes. It will be observed from the figure that the true vil- 
lous chorion is confined to a comparatively small region; there 
is, however, in addition a false chorion without villi, but highly 
vascular. This blending of forms of placentation which exist 
separately in different groups of animals is significant. In the 
rabbit, at a later stage, there is considerable intermingling of 
foetal and maternal parts. 
The Metadiscoidal Placenta.— This type, which, in general 
naked-eye appearances, greatly resembles the former, is found 
in man and the apes. The condition of things in man is by no 
means as well understood as in the lower mammals, especially 
in the early stages; so that, while the following account is that 
usually given in works on embryology, the student may as well 
understand that our knowledge of human embryology in the 
very earliest stages is incomplete and partly conjectural. The 
reason of this is obvious: specimens for examination depending 
on accidents giving rise to abortion or sudden death, often not 
reaching the laboratory in a condition permitting of trust- 
worthy inferences. 
It is definitely known that the ovum, which is usually fer- 
tilized in the oviduct (Fallopian tube), on entering the uterus 
\ becomes adherent to its wall and encapsuled. The mucous 
membrane of the uterus is known to undergo changes, its com- 
ponent parts increasing by cell multiplication, becoming in- 
tensely vascular and functionally more active. The general 
mucous surface shares in this, and is termed the decidua vera ; 
but the locality where the ovum lodges is the seat of the great- 
est manifestation of exalted activity, and is termed the decidua 
serotina ; while the part believed to have invested the ovum by 
fused growths from the junction of the decidua vera and sero- 
tina is the decidua reflexa. 
6 
