88 ANIMAL PHYSLOLOGY. 
tissue, in which capillaries abound, are covered with a flat epi- 
thelium; the maternal crypts correspond, being composed of 
a similar matrix, lined with epithelium and permeated by 
capillary vessels, which constitute a plexus or mesh-work, It 
thus results that two layers of epithelium intervene between 
the maternal and foetal capillaries. 
The arrangement is substantially the same in the diffuse and 
the cotyledonary placenta. 
In the deciduate placenta, naturally, there is greater compli- 
cation. 
In certain forms, as in the fox and cat, the maternal tis- 
sue shows a system of trabeculae assuming a meshed form, 
in which run dilated capillaries. These, which are covered 
with a somewhat columnar epithelium, are everywhere in 
contact with the foetal villi, which are themselves covered with 
a flat epithelium. 
a 
(@ 
Vie 
Fig. 100. Fig. 101. 
Fre. 100.—Placenta of a sloth. Flay maternal epithelial cells shown in position on right side ; 
on left they are removed and dilated ; maternal vessel with its blood-corpuscles exposed. 
Fic. 101.—Structure of human placenta : ds, decidua serotina ; t, trabecule of serotina passin; 
to foetal villi; ca, curling artery ; up, utero-placental vein; 2, prolongation of maternal 
tissue on exterior of villus, outside cellular layer e’, which may represent either endothe- 
lium of maternal blood-vessels or delicate connective tissue of the serotina or both ; e’ ma- 
ternal cells of the serotina. 
In the case of the sloth, with a more discoidal placenta, the 
dilatation of capillaries and the modification of epithelium 
are greater. 
In the placenta of the apes and of the human subject the 
most marked departure from simplicity is found. The maternal 
