About the Malpighian follicles of several species of rodents, and especially prominent in rats, is a perifollicular envelope composed of hematopoietic cells that takes active part in hemolytopoietic changes.
The identity of the mononuclear cell comprising the greater part of this tissue has not been positively determined; it is probably a young lymphocyte. It is possibly the homologue of the pale centers of the Malpighian follicles in man and other mammals, though it has been found in rabbit spleens which also have pale centers. It should be possible to determine this identity by the use of a greater variety of stains, and of test poisons, and, if possible, of dynamic methods such as tissue culture and moving pictures.
The envelope is separated from the Malpighian follicle by a thin rind of collagenous connective tissue, but on its outer margin it merges gradually with the red pulp. It often contains a scattering of erythrocytes, normoblasts, polymorphonuclear neutrophils, and rarely eosinophils and pigment-bearing macrophages. Some of these cells were so greatly increased under the pathologic conditions first studied that colonization was suggested; they were later thought to have probably wandered in. The collar never contains megakaryocytes or sinuses or blood vessels of any noteworthy size.