The chief part of the oval embryonic shield is at first the narrow hinder end; it is in the middle line of this that the primitive streak appears (Fig. 121 _ps_).The narrow longitudinal groove in it—the so-called “primitive groove”—is, as we have seen, the primitive mouth of the gastrula. In the gastrula-embryos of the mammals, which are much modified cenogenetically, this cleft-shaped prostoma is lengthened so much that it soon traverses the whole of the hinder half of the dorsal shield; as we find in a rabbit embryo of six to eight days (Fig. 122 _pr_). The two swollen parallel borders that limit this median furrow are the side lips of the primitive mouth, right and left. In this way the bilateral-symmetrical type of the vertebrate becomes pronounced. The subsequent head of the amniote is developed from the broader and rounder fore-half of the dorsal shield. In this fore-half of the dorsal shield a median furrow quickly makes its appearance (Fig. 123 _rf_). This is the broader dorsal furrow or medullary groove, the first beginning of the central nervous system. The two parallel dorsal or medullary swellings that enclose it grow together over it afterwards, and form the medullary tube. As is seen in transverse sections, it is formed only of the outer germinal layer (Figs. 95 and 136). The lips of the primitive mouth, however, lie, as we know, at the important point where the outer layer bends over the inner, and from which the two cœlom pouches grow between the primary germinal layers. Fig.121. Oval embryonic shield of the rabbit. Fig. 121—Oval embryonic shield of the rabbit (_A_ of six days eighteen hours, _B_ of eight days). (From _Kölliker._) _ps_ primitive streak, _pr_ primitive groove, _arg_ area germinalis, _sw_ sickle-shaped germinal growth. Fig.122. Dorsal shield (ag) and germinative area of a rabbit-embryo of eight days. Fig. 123. Embryonic shield of a rabbit of eight days. Fig. 122—Dorsal shield (_ag_) and germinative area of a rabbit-embryo of eight days. (From _Kölliker._) _pr_ primitive groove, _rf_ dorsal furrow. Fig. 123.—Embryonic shield of a rabbit of eight days. (From _Van Beneden._) _pr_ primitive groove, _cn_ canalis neurentericus, _nk_ nodus neurentericus (or “Hensen’s ganglion”), _kf_ head-process (chorda). Thus the median primitive furrow (_pr_) in the hind-half and the median medullary furrow (_Rf_) in the fore-half of the oval shield are totally different structures, although the latter seems to a superficial observer to be merely the forward continuation of the former. Hence they were formerly always confused. This error was the more pardonable as immediately afterwards the two grooves do actually pass into each other in a very remarkable way. The point of transition is the remarkable neurenteric canal (Fig. 124 _cn_). But the direct connection which is thus established does not last long; the two are soon definitely separated by a partition.