The first or primitive trace of the embryo is in the cicatricula or germinal membrane, which contained the germinal vesicle before its disappearance. In the centre of this, upon its upper surface, may be discovered a small dark line;[25] "this line or primitive trace is swollen at one extremity, and is placed in the direction of the transverse axis of the egg." [Illustration: _a_ Transparent area. _b_ Primitive trace.] As development advances, the cicatricula expands. "We are indebted to Pander,"[26] says Dr. Allen Thomson in his admirable essay above quoted, "for the important discovery, that towards the twelfth or fourteenth hour, in the hen's egg the germinal membrane becomes divided into two layers of granules, the serous and mucous layers of the cicatricula; and that the rudimentary trace of the embryo, which has at this time become evident, is placed in the substance of the upper-most or serous layer." "According to this observer, and according to Baer, the part of this layer which surrounds the primitive trace soon becomes thicker; and on examining this part with care, towards the eighteenth hour, we observe that a long furrow has been formed in it, in the bottom of which the primitive trace is situated; about the twentieth hour this furrow is converted into a canal open at both ends, by the junction of its margins (the _plicæ primitivæ_ of Pander, the _laminæ dorsales_ of Baer:) the canal soon becomes closed at the cephalic or swollen extremity of the primitive trace, at which part it is of a pyriform shape, being wider here than at any other part. According to Baer and Serres, some time after the canal begins to close, a semi-fluid matter is deposited in it, which on its acquiring greater consistence, becomes the rudiment of the spinal cord; the pyriform extremity or head is soon after this seen to be partially subdivided into three vesicles, which being also filled with a semi-fluid matter, gives rise to the rudimentary state of the encephalon." "As the formation of the spinal canal proceeds, the parts of the serous layer which surrounds it, especially towards the head, become thicker and more solid, and before the twenty-fourth hour we observe on each side of this canal four or five small round opaque bodies, these bodies indicate the first formation of the dorsal vertebræ. [Illustration: _a_ Transparent area. _b_ Laminæ dorsales. _c_ Cephalic end. _d_ Rudiments of dorsal vertebræ. _e_ Serous layer. _f_ Lateral portion of the primitive trace. _g_ Mucous layer. _h_ Vascular layer. _k_ Laminæ dorsales united to form the spinal canal.]