Fig.64. Blastula of the opossum (Didelphys). Fig. 64—Blastula of the opossum (_Didelphys_). (From _Selenka._) _a_ animal pole of the blastula, _ v_ vegetal pole, _en_ mother-cell of the entoderm, _ ex_ ectodermic cells, _s_ spermia, _ib_ unnucleated yelk-balls (remainder of the food-yelk), _p_ albumin membrane.
The marsupials, a second sub-class, come next to the oviparous monotremes, the oldest of the mammals. But as in their case the food-yelk is already atrophied, and the little ovum develops within the mother’s body, the partial cleavage has been reconverted into total. One section of the marsupials still show points of agreement with the monotremes, while another section of them, according to the splendid investigations of Selenka, form a connecting-link between these and the placentals.
The fertilised ovum of the opossum (_Didelphys_) divides, according to Selenka, first into two, then four, then eight equal cells; hence the segmentation is at first equal or homogeneous. But in the course of the cleavage a larger cell, distinguished by its less clear plasm and its containing more yelk-granules (the mother cell of the entoderm, Fig. 64 _en_), separates from the others; the latter multiply more rapidly than the former. As, further, a quantity of fluid gathers in the morula, we get a round blastula, the wall of which is of varying thickness, like that of the amphioxus (Fig. 38 _E_) and the amphibia (Fig. 45). The upper or animal hemisphere is formed of a large number of small cells; the lower or vegetal hemisphere of a small number of large cells. One of the latter, distinguished by its size (Fig. 64 _en_), lies at the vegetal pole of the blastula-axis, at the point where the primitive mouth afterwards appears. This is the mother-cell of the entoderm; it now begins to multiply by cleavage, and the daughter-cells (Fig. 65 _ i_) spread out from this spot over the inner surface of the blastula, though at first only over the vegetal hemisphere. The less clear entodermic cells (_i_) are distinguished at first by their rounder shape and darker nuclei from the higher, clearer, and longer entodermic cells (_e_), afterwards both are greatly flattened, the inner blastodermic cells more than the outer.