The Corpus Luteum (a) Follicular Wall--The membrana granulosa in the three species, P. cinereus, T. vulpecula, and D. aurita, is composed of typical polygonal cells arranged three or four cells deep around the ripe follicle. The theca folliculi also calls for no special comment in any case. It is composed of internal and external layers, does not contain any included interstitial cells, and its cells are always readily distinguishable from membrana granulosa cells. (b) The Formation of the Corpus Luteum--The corpus luteum in P. cinereus is formed by the irruption of both layers of the theca folliculi, which burst through the membrana granulosa and form a lining on its inner side. This method of formation is similar to that in P. obesula, P. nasuta, and M. ruficollis. The ripe follicle in T. vulpecula collapses when the ovum is extruded, and the central cavity is at once obliterated. The theca folliculi is drawn in with the membrana granulosa, which it penetrates, and the connective tissue becomes irregularly distributed through the body. It is unlike the process in any other marsupial so far examined, but to a certain extent resembles that in the mouse. In D. aurita the thecal irruptions do not at once go through the membrana granulosa, but push it before them until the central cavity is practically filled in, and then they break through and form the central plug of connective tissue. In one example, a very early stage, mitoses were found in the cells of the membrana granulosa, as was also the case in P. obesula and P. nasuta. (c) The fully formed Corpus Luteum.--The corpus luteum in P. cinereus remains hollow even when fully grown, and the central cavity does not get filled in until some time after the birth of the young, apparently not until the gland has started to decline. This condition is apparently unique. In T. vulpecula the corpus luteum is fairly typical when full grown, save that its connective tissue is much more irregularly arranged than in other marsupials. The condition of the corpus in D. aurita is very similar to that in D. viverrinus In no case is the membrana granulosa shed, nor does the theca interna contribute to the lutein cells of the corpus luteum. The Interstitial Tissue. There is present in the ovary of certain species of marsupials a tissue which corresponds histologically to the interstitial tissue in the ovary of the higher mammals. The cells are always distinguishable from ordinary stroma cells, cells of the theca interna, old lutein cells, or the cells of an atresic follicle, and there is no evidence that any of the last three are at any time transformed into interstitial cells. Such cells are present in the pouch young of T. vulpecula before they could have been derived from any of the sources suggested above. Interstitial tissue is to be regarded as a tissue suigeneris, although it is possible that it may originate from modified stroma cells at a very early stage. The tissue is irregularly distributed in the various species of marsupials, and it is worthy of note that it is present in all the Diprotodontia and absent in the Polyprotodontia so far examined. It may be present only as a few small groups of cells or in such quantity as to form by far the largest part of the bulk of the ovary, excluding corpora lutea, as, for example, in P. penicillata. The tissue has a typical glandular appearance, but no satisfactory account of its function has yet been put forward, and in view of this and its irregularity it is preferable not to call it a gland, but retain the term interstitial tissue or cells.