Thus the gastrulation of the placentals, which diverges most from that of the amphioxus, the primitive form, is reduced to the original type, the invagination of a modified blastula. Its chief peculiarity is that the folded part of the blastoderm does not form a completely closed (only open at the primitive mouth) blind sac, as is usual; but this blind sac has a wide opening at the ventral curve (opposite to the dorsal mouth); and through this opening the primitive gut communicates from the first with the embryonic cavity of the blastula. The folded crest-shaped entoderm grows with a free circular border on the inner surface of the entoderm towards the vegetal pole; when it has reached this, and the inner surface of the blastula is completely grown over, the primitive gut is closed. This remarkable direct transition of the primitive gut-cavity into the segmentation-cavity is explained simply by the assumption that in most of the mammals the yelk-mass, which is still possessed by the oldest forms of the class (the monotremes) and their ancestors (the reptiles), is atrophied. This proves the essential unity of gastrulation in all the vertebrates, in spite of the striking differences in the various classes. Fig.68. Stem-cell of the mammal ovum (from the rabbit). Fig. 69. Incipient cleavage of the mammal ovum (from the rabbit). Fig. 70. The first four segmentation-cells of the mammal ovum (from the rabbit). Fig. 71. Mammal ovum with eight segmentation-cells (from the rabbit). Fig. 68—Stem-cell of the mammal ovum (from the rabbit). _k_ stem-nucleus, _n_ nuclear corpuscle, _p_ protoplasm of the stem-cell, _z_ modified zona pellucida, _h_ outer albuminous membrane, _s_ dead sperm-cells. Fig. 69 Incipient cleavage of the mammal ovum (from the rabbit). Fig. 69—Incipient cleavage of the mammal ovum (from the rabbit). The stem-cell has divided into two unequal cells, one lighter (_e_) and one darker (_i_). _z_ zona pellucida, _h_ outer albuminous membrane, _s_ dead sperm-cell. Fig. 70 The first four segmentation-cells of the mammal ovum (from the rabbit). Fig. 70—The first four segmentation-cells of the mammal ovum (from the rabbit). _e_ the two larger (and lighter) cells, _i_ the two smaller (and darker) cells, _z_ zona pellucida, _h_ outer albuminous membrane. Fig. 71 Mammal ovum with eight segmentation-cells (from the rabbit). Fig. 71—Mammal ovum with eight segmentation-cells (from the rabbit). _e_ four larger and lighter cells, _i_ four smaller and darker cells, _z_ zona pellucida, _h_ outer albuminous membrane.