cells being loose there. These form the yelk-stopper, and fill up the mouth of the gastrula (_a_). The central primitive gut-cavity (_d_) is full of entodermic cells. The uni-axial type of the mammal gastrula is accentuated in this way. However, opinions still differ considerably as to the real nature of this “provisional gastrula” of the placental and its relation to the blastula into which it is converted. As the gastrulation proceeds a large spherical blastula is formed from this peculiar solid amphigastrula of the placental, as we saw in the case of the marsupial. The accumulation of fluid in the solid gastrula (Fig. 73 A) leads to the formation of an eccentric cavity, the group of the darker entodermic cells (_hy_) remaining directly attached at one spot with the round enveloping stratum of the lighter ectodermic cells (_ep_). This spot corresponds to the original primitive mouth (prostoma or blastoporus). From this important spot the inner germinal layer spreads all round on the inner surface of the outer layer, the cell-stratum of which forms the wall of the hollow sphere; the extension proceeds from the vegetal towards the animal pole. Fig.67. Longitudinal section through the oval gastrula of the opossum. Fig. 67—Longitudinal section through the oval gastrula of the opossum (Fig. 69). (From _Selenka._) _p_ primitive mouth, _e_ ectoderm, _i_ entoderm, _d_ yelk remains in the primitive gut-cavity (_u_). The cenogenetic gastrulation of the placental has been greatly modified by secondary adaptation in the various groups of this most advanced and youngest sub-class of the mammals. Thus, for instance, we find in many of the rodents (guinea-pigs, mice, etc.) _ apparently_ a temporary inversion of the two germinal layers. This is due to a folding of the blastodermic wall by what is called the “girder,” a plug-shaped growth of Rauber’s “roof-layer.” It is a thin layer of flat epithelial cells, that is freed from the surface of the blastoderm in some of the rodents; it has no more significance in connection with the general course of placental gastrulation than the conspicuous departure from the usual globular shape in the blastula of some of the ungulates. In some pigs and ruminants it grows into a thread-like, long and thin tube.