The capillary endothelium is separated from the syntrophoblast by an amorphous layer known as the (1) (see Fig. 40); it is thought that this layer is derived from the endothelial basal lamina. The (1) becomes more distinct (histologically) as gestation advances. Occasionally, maternal fibroblasts remain between the endothelium and the (1). These cells are known as (2) cells and are small and inconspicuous in the dog (see Fig. 40). However, in the cat, they are large, swollen polygonal cells containing (3).