

FORMATION OF THE EMBRYO. 731 



cells of the lower layer are rather larger, more globular in form, and less 

 closely united with each other. The whole forms an organized cellular 

 membrane, the blastoderm, which, occupies the place of the original 

 cicatricula. 



Thus the blastoderm, or germinal membrane, is formed in the impreg- 

 nated fowl's egg by a process of segmentation essentially similar to that 

 which takes place in eggs of other kinds. It presents the appearance of 

 a thin sheet, of uniform texture, composed of nothing but cells, lying 

 at one spot upon the surface of the yolk. Its formation, which begins 

 immediately after the impregnation of the egg, continues, under the 

 influence of the animal temperature, during the eighteen or twenty hours 

 that the egg is retained in the oviduct for the deposit of its albumen and 

 external envelopes. According to Foster and Balfour, it has reached the 

 condition of a distinct cellular membrane at the time of the expulsion 

 of the egg. If afterward kept at a low temperature it remains in this 

 state ; but, if subjected to natural or artificial incubation at a tempera- 

 ture of 38 (100 F.), it goes on to the further development of the body 

 of the embryo. 



Folds of the Blastoderm. The form of the body of the embryo and 

 of its different parts is sketched out, in all cases, by means of a series 

 of folds, which show themselves at various points in the blastoderm. 

 This membrane presents at first a flat surface ; or, i'f it have a certain 

 degree of convexity, corresponding with that of the } T olk upon which it 

 lies, this convexity is perfectly uniform, and is too slightly pronounced 

 to be appreciable within the limits of the blastoderm. But as soon as 

 development begins to make a definite progress, this uniformity of sur- 

 face is broken by the appearance of folds or ridges, which are directed 

 longitudinally or transversely, and which thus mark the lines of separa- 

 tion between different parts of the blastoderm. Such a fold, running in 

 a curvilinear direction from side to side, marks the position of the head 

 of the embryo, and is called the " head-fold." The free border of this 

 mass, projecting forward and above the neighboring portion of the blas- 

 toderm, becomes in fact the head, which, as well as the neck, is curved 

 more and more forward and downward, in the subsequent stages of 

 embryonic growth, with the deepening of the fold which first gave origin 

 to it as a distinct part. A similar transverse curvilinear fold at the 

 posterior portion of the area pellucida, marks off the hinder extremity 

 of the embryo, and is called the " tail-fold." Longitudinal folds are 

 also formed in the same manner, one on each side, which fix the lateral 

 limits of the body of the embryo. 



By this means, a certain portion of the blastoderm becomes distinctly 

 marked off from the rest. The part included within the transverse and 

 longitudinal folds is immediately recognizable as the body of the embryo; 

 while that which remains outside these limits becomes developed into 

 accessory organs, playing an important though secondary part in the 

 history of development. This forms a marked distinction between the 

 process as it takes place in the fowl's egg, and that already described in 



