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.
Fig.124. Longitudinal section of the coelomula of amphioxus. Fig. 124—Longitudinal section of the cœlomula of amphioxus (from the left). _i_ entoderm, _d_ primitive gut, _cn_ medullary duct, _n_ nerve tube, _m_ mesoderm, _s_ first primitive segment, _c_ cœlom-pouches. (From _Hatschek._)