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b.) The Guinea-Pig. 



Our knowledge of the placenta of this Rodent is entirely due 

 to Duval (6). 



In the position and shape of the placenta, and in the arrange- 

 ment of the foetal membranes the Guinea-Pig is quite similar to 

 the mouse, the only difference being that the distal wall of the 

 yolk-sac is never formed at all. 



An ectoplacenta is formed which has the shape of a concave 

 disc. A small central part remains throughout unaltered, but the 

 annular peripheral portion becomes enormously thickened. The 

 superficial part of this latter is vascularised by the capillaries of 

 the allantois (piasmode remanié') the deeper part remaining in 

 its original condition ('piasmode primitif). By the excavation of 

 'lacunes sanguimaternelles' in the ectoplacenta, the maternal and 

 foetal blood channels are brought into intimate relation in the 

 ordinary way. 



The bounding surface between these two parts is uneven, and 

 in the course of gestation they come to interdigitate in such a 

 way that lobules of the 'piasmode remanié' are formed separated 

 from one another by the interlobular tissue of the 'piasmode 

 primitif'. 



The lobules contain the maternal main arterial channels, passing 

 directly to the foetal side of the placenta, and the principal bran- 

 ches of the umbilical veins. The interlobular tissue similarly con- 

 tains the chief maternal efferent, and foetal afferent blood spaces. 

 The reciprocal relations of the two in the Guinea-Pig are there- 

 fore identical with the arrangement which obtains in the Mouse 

 and Rabbit. Tafani (49) has also found this to be so. 



The cells in the deepest portions of the ectoplacenta undergo 

 a trausformation into megalokaryocytes ; and Duval has also des- 

 cribed, not very clearly, an endovascular plasmodium. He has 

 only shown that plasmodial processes penetrate maternal tissue, 

 not that they enter the blood vessels. 



Claude Bernard (4) mentions the presence of a layer of gly- 

 cogenic tissue on the maternal side of the placenta. 



