ONTOGENESIS 



225 



concavo-convex plate, the convex surface of which con- 

 sisting of ectoderm is destined to form the dorsum and 

 external coverings of the embryo, the concave surface 

 the internal organs about which the embryo is to grow. 

 As the growth proceeds, the circumference of the germinal 



Medullary 

 folds 



Medullary 

 furrow 



Primitive streak 

 and groove 



FIG. 88. Surface view of area pellucida of an eighteen-hour chick embryo. 

 The area opaca is omitted; the pear-shaped outline marks the limit of the area 

 pellucida. At the place where the two medullary folds are continuous with 

 each other there is to be seen a short curved line, which represents the head 

 fold. In front of it there lies a second line concentric with it, the beginning 

 of the amniotic fold. (Balfour.) 



disc increases, and the subjacent yolk is absorbed as it 

 furnishes the growing cells with nourishment. The 

 increasing* disc does not grow uniformly and hence be- 

 comes thrown into folds which, when viewed from the 

 dorsal surface, indicate the position of future structures. 

 Thus an anterior transverse fold indicates where the am- 

 niotic membrane is to form; a second t^isverse fold, 

 where the head of the embryo is to develop. The longi- 

 tudinal groove in the anterior part of the disc indicates 



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