362 GENERATION. 



development, it is observed that a portion of the blastoderm 

 becomes thickened, to form the first trace of the embryon. 

 At this portion, where the body of the embryon subsequently 

 makes its appearance, as we have already seen, we have the 

 external layer, the internal layer, and a thick, intermediate 

 layer of cells, developed from the opposite surfaces of the 

 external and the internal layer, called the middle layer. At 

 nearly the time when this thickening begins, a fold of the 

 external layer makes its appearance, surrounding the thick- 

 ened portion, and most prominent at the cephalic and the 

 caudal extremity of the furrow for the neural canal. This 

 fold increases in extent as development advances, passes over 

 the dorsal surface of the embryon, and finally meets so as to 

 enclose the embryon completely. We can readily figure to 

 ourselves this process and understand how, at a certain period 

 of the development of the amnion, this membrane consists 

 of an external layer, formed of the external layer of the fold, 

 and an internal layer; the point of union of the two layers, 

 or the point of meeting of the fold, being marked by a mem- 

 branous septum. An illustration of this mode of formation of 

 the two layers of the amnion is afforded by an observation by 

 Allen Thomson, made on an embryon of about fifteen days, 

 in which there was a portion of membrane extending from the 

 internal amniotic fold to the external covering of the ovum. 1 



scription of the mode of formation of the membranes so clear and exact as that 

 given by Dalton, in his Treatise on Physiology. In our account of these pro- 

 cesses, we shall follow in the main this description. (DALTON, Human Physi- 

 ology, Philadelphia, 1871, p. 619, et seq.) 



1 ALLEN THOMSON, Contributions to the History and Structure of the Human 

 Ovum, etc. Edinburgh Medical and Surgical Journal, 1839, vol. iii., p. 132. 

 This observation is quoted by Longet as showing the mechanism of formation 

 of the amnion (Traite de physiologic, Paris, 1869, tome iii., p. 844); but the 

 statement in the original paper by Allen Thomson is rather indefinite. Allud- 

 ing to " a portion of fine membrane which adhered to the notch between the 

 foetus and yolk at the cephalic extremity," he says : " I am inclined to believe 

 that this piece of membrane may be a part of the cephalic fold of the serous 

 layer of the germinal membrane which forms the amnios." The ovum was not 

 examined until after it had been in alcohol for several days. 



