A system of midwifery . cular appearance, which we have already mentioned when de-scribing this membrane. The first or primitive trace of the embryo is in the cicatricula or ger-minal membrane, which contained the germinal vesicle before its disap-pearance. In the centre of this, upon its upper surface, may be discovereda small dark line :t This line or primitive trace is swollen at one extremi-ty, and is placed in the direction of the transverse axis of the egg. As development advances, the cicatricula expands. We are indebt-ed to Pander,! says Dr. Allen Thomson in his admirable essay above *
A system of midwifery . cular appearance, which we have already mentioned when de-scribing this membrane. The first or primitive trace of the embryo is in the cicatricula or ger-minal membrane, which contained the germinal vesicle before its disap-pearance. In the centre of this, upon its upper surface, may be discovereda small dark line :t This line or primitive trace is swollen at one extremi-ty, and is placed in the direction of the transverse axis of the egg. As development advances, the cicatricula expands. We are indebt-ed to Pander,! says Dr. Allen Thomson in his admirable essay above * We said, one of the earliest changes. Mr. Jones considers that the breaking upof the surface of the yelk into crystalline forms, is the first change which he has ob-served. t Allen Thomson on the Development of the Vascular System in the Foetus of Verte-brated Animal. (Edin. New Fhi/osop. Journ. Oct. 1830.^ X Pander, Beitrage zur Entwickelungs-geschichte des Hiinchens im Eie. Wiirzburg,1817. 62 DEVELOPMENT OF THE quoted, for the important discovery, that towards the twelfth or four-teenth hour, in the hens egg, the germinal membrane becomes dividedinto two layers of granules, the serous and mucous layers of the cicatricu- la; and that the rudimentary traceof the embryo, which has at thistime become evident, is placed inthe substance of the uppermost orserous layer. According to thisobserver, and according to Baer,the part of this layer which sur-rounds the primitive trace soon be-comes thicker; and on examining a Transparent area. ft Primitive trace, this part with Care, tOWards the eighteenth hour, we observe that a long furrow has been formed in it, inthe bottom of which the primitive trace is situated ; about the twentiethhour this furrow is converted into a canal open at both ends, by the junc-tion of its margins (the plicce primitivcB of Pander, the lamince dor salesof Baer:) the canal soon becomes closed at the cephalic or swollen extre-mity of the primitive trace
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