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Show 1 9 0 5 .] PLACENTA OF THE SPINY MOUSE. 2 8 7 large efferent vessels which presumably debouch into two maternal veins (MY). These large efferent maternal vessels are more peripheral in position than the afferent courses. The foetal blood-supply is less easily followed. The allantoic arteries, on reaching the surface of the placenta, radiate and subdivide and give off branches which pass into the placenta at intervals over the more central part of the surface. These pass (as regards the main stream) straight through the fretal half of the placenta, but give off at frequent intervals small capillaries, which take a more sinuous and radial course, anastomosing, forming network, and collecting together again into veins, which I believe to run parallel with the arteries of the villus. A considerable amount of connective tissue accompanies these villi for the first part of their ingress into the placenta. Comparison with other Placentas. The placenta is clearly of the type which I have described elsewhere as cumulate as contrasted with the plicate type characteristic of such groups as the Ungulates. The general shape, the arrangement of the membranes and their character, resemble, according to the interpretation placed upon them above, the condition that pertains in Mus musculus, and, rather less closely, that of Mus decumanus. Jenkinson has recently given an account of the development of the placenta of the Mouse, which differs in several important particulars from that of Duval, which, until that time, had been generally accepted. Jenkinson agrees with Duval as regards the general arrangement of membranes and in the main features of the development. He differs, however, in respect to the vascular system, and my account given above is in complete agreement with Jenkinson. I see also nothing to suggest the appearance of an ingrowth of the trophoblast into the maternal blood-vessels, and growth along' the inside as described by Duval (" plasmode endovasculaire "). Another point of disagreement is with reference to the glycogenous tissue. Duval takes but little notice of this and does not seem to have found the maternal glycogenous mass, which, according to Jenkinson, degenerates, and the space occupied by it becomes subsequently filled with a second glycogenous tissue which is of foetal (trophoblastic) origin. This is a matter of very considerable interest. It is not to be expected that an isolated case in an allied genus can afford any conclusive evidence. Unfortunately the method of preservation (corrosive sublimate) is not suitable to the study of glycogen. Tested with iodine I find no trace of glycogen, but there are certain spaces in the trophoblast cells alluded to in the foregoing |