REPRODUCTIOIir. 89 



placenta are reduced to three, two arteries and one vein. The 

 villi of the placenta (chorion) are usually said to hang freely 

 in the hlood of the large irregular sinuses of the decidua sero- 

 tina; but this is so unlike what prevails in other groups of 

 animals that we can not refrain from believing that the state- 

 ment is not wholly true. 



The Zonary Placenta. — In this type the pleicenta is formed 

 along a broad equatorial belt, leaving the poles free. This form 

 of placentation is exemplified in the carnivora, hyrax, the ele- 

 phant, etc. 



In the dog, for example, the yelk-sac is large, vasciilar, does 

 not fuse with the chorion, and persists throughout. A rudi- 

 mentary discoid placenta is first formed, as in the rabbit; this 

 gradually spreads over the whole central area, till only the ex-, 

 tremes (poles) of the ovum remain free; villi appear, fitting into 

 pits in the uterine surface, the maternal and foetal parts of the 

 placenta becoming highly vascular and closely approximated. 

 The chorionic zone remains wider than the placental. As in 

 man there is at birth a separation of the maternal as well as 

 foetal part of the placenta — i. e., the latter is deciduate; there is 

 also the beg^ning of a decidua reflexa. 



The Diffuse Placenta. — As found in the horse, pig, lemur, 

 etc., the allantois completely incloses the embryo, and it be- 

 comes villous in all parts, except a small area at each pole. 



The Polycotyledonary Placenta.— This form is that met with 

 in ruminants, in which case the allantois completely covers the 

 surface of the subzonal membrane, the placental villi being 

 gathered into patches {chorial cotyledons), which are equivalent 

 to so many independent placentas. The component villi fit into 

 corresponding pits in the uterine wall (uterine cotyledons), 

 which is specially thickened at these points. When examined 

 in a fresh condition, under water, they constitute very beautiful 

 objects. The pits referred to above into which the foetal villi fit 

 are, as shown in the figures on page 91, essentially the same in 

 structure as the villi themselves. In the cow the uterine cotyle- 

 dons are convex ; but in the sheep and goat they are raised con- 

 cave cups in which the openings for the foetal villi may be seen 

 with the naked eye. The differences are not essential ones. 



Between the uterine cotyledons and the foetal villi which 

 fit into them a thickish, mUky-looking fluid is found, the 

 " uterine milk " elaborated, no doubt, by the cells which line the 

 cotyledonous pits. 



