We may now mention certain facts which are not so generally used. The partly primitive structure of the right auriculo-ventricular valve in the Monotremata has no counterpart in any Marsupial which has been dissected; but there are traces in the latter of the characteristic "ventral mesentery" of _Ornithorhynchus_ and _Echidna_.[67] Mr. Caldwell's interesting observation upon the segmenting egg of the Marsupial, the incompleteness of the first segmentation furrow (reminding us of the meroblastic ovum of the Monotreme), may possibly not turn out to be so exclusively Marsupial a feature as has been thought. The balance of evidence thus points to the nearer relationship of the Marsupials to the Eutherian mammals; and their great specialisation combined with certain evidences of degeneration (disappearance in part of the milk dentition), and their age, point to the fact that they are, at any rate, the descendants of an early form of Eutherian. But they must have separated from the Eutherian stock after it had acquired a definite diphyodonty and the allantoic placenta, the two principal features of the Eutherian as opposed to the Prototherian mammals. Nevertheless it seems probable that the Marsupial tribe is derived from some of the earliest Eutherians. And on this view may be explained the retention of Prototherian characters. The remaining Eutheria are obviously all to be referred to one great division with the possible exception of the Whales, whose affinities form one of the principal difficulties to the student {120} of this group. A short resume of what is at present thought of the systematic position of this anomalous order is appropriate here. Albrecht went so far as to regard the Cetacea as the nearest group of animals to the hypothetical Promammalia.[68] But discounting his arguments by the removal of such of them as relate to structure plainly altered by the singular mode of life of these creatures, there is really a great deal to be said in favour of his view.