TEM).
Microscopically the spleen,hias a fibro-elastic coat in which in voluntary muscle is found (fig. 2) . This coat sends fine trabeculae into the organ, subdividing it into minute compartments, in which the red, highly vascular, spleen pulp is contained. This pulp con tains small spherical masses of adenoid tissue (Malpighian cor puscles), situated on the terminal branches of the splenic blood vessels, numerous red blood corpuscles, lymphocytes and endo thelial cells, the last often containing pigment granules or fat. The arteries in large part open into spaces; which give origin to the veins.
The spleen is developed in the dorsal mesogas trium (see COELOM AND SEROUS MEMBRANES) from the mesen chyme, or that portion of the mesoderm, the cells of which lie scattered in a matrix. Large lymphoid cells are early seen among those of the mesenchyme, probably being derived from the coelomic epithelium. The network of the spleen seems certainly to be derived from cells of the mesenchyme which lose their nuclei.
The spleen is regarded as the re mains of a mass of lymphoid tissue which, in a generalized type of vertebrate, stretched all along the alimentary canal. It is absent as a distinct gland in the Acrania and Cyclostomata. In the fishes it is closely applied to the U-shaped stomach, and in some of the Elasmobranchs, e.g., the basking and porbeagle sharks, it is di vided into small lobules. In Protopterus (Dipnoi) it is enclosed within the walls of the stomach. In frogs and toads (Amphibia) it is a spherical mass close to the rectum, and is derived from a different part of the original mass, already mentioned, to that which persists in other vertebrates. In the Iguana (Reptilia) the organ has many notches, each corresponding to the point of en trance of a vessel. In mammals the notches, when present, also frequently correspond to the points of entrance of arteries at the hilum. The Monotremata and Marsupialia have curious Y-shaped spleens. As a rule flesh-eating animals have larger and more notched spleens than vegetable feeders, though among the Cetacea the spleen is relatively very small.
