During these processes important changes are taking place in the outer germinal layer (the “skin-sense layer”). The continued rise and growth of the dorsal swellings causes their higher parts to bend together at their free borders, approach nearer and nearer (Fig. 136 _w_), and finally unite. Thus in the end we get from the open dorsal furrow, the upper cleft of which becomes narrower and narrower, a closed cylindrical tube (Fig. 137 _mr_). This tube is of the utmost importance; it is the beginning of the central nervous system, the brain and spinal marrow, the _medullary tube._ This embryonic fact was formerly looked upon as very mysterious. We shall see presently that in the light of the theory of descent it is a thoroughly natural process. The phylogenetic explanation of it is that the central nervous system is the organ by means of which all intercourse with the outer world, all psychic action and sense-perception, are accomplished; hence it was bound to develop originally from the outer and upper surface of the body, or from the outer skin. The medullary tube afterwards separates completely from the outer germinal layer, and is surrounded by the middle parts of the provertebræ and forced inwards (Fig. 146).The remaining portion of the skin-sense layer (Fig. 93 _h_) is now called the horn-plate or horn-layer, because from it is developed the whole of the outer skin or epidermis, with all its horny appendages (nails, hair, etc.). A totally different organ, the _prorenal_ (primitive kidney) _duct_ (_ung_), is found to be developed at an early stage from the ectoderm. This is originally a quite simple, tube-shaped, lengthy duct, or straight canal, which runs from front to rear at each side of the provertebræ (on the outer side, Fig. 93 _ung_). It originates, it seems, out of the horn-plate at the side of the medullary tube, in the gap that we find between the provertebral and the lateral plates. The prorenal duct is visible in this gap even at the time of the severance of the medullary tube from the horn-plate. Other observers think that the first trace of it does not come from the skin-sense layer, but the skin-fibre layer. Fig.135. Sandal-shaped embryonic shield of an opossum. Fig. 135—Sandal-shaped embryonic shield of an opossum (_Didelphys_), three days old. (From _Selenka._) (Back view from above.) _stz_ stem-zone or dorsal shield (with eight pairs of primitive segments), _pz_ parietal or ventral zone, _ap_ pellucid area, _ao_ opaque area, _hh_ halves of the heart, _v_ fore-end, _h_ hind-end. In the median line we see the chorda (_ch_) through the transparent medullary tube (_m_). _u_ primitive segment, _pr_ primitive streak (or primitive mouth).