212 BIOLOGY: GENERAL AND MEDICAL 



the blastula is perfected, gastrulation takes place not 

 by the simple invagination of one side of the sphere, 

 for the segmentation cavity is not spherical, but through 

 a combination of folding and invagination by which one 

 portion, the outer layer, forming an ectoderm, comes to 

 overlie the inner layer, the entoderm, and form a kind 

 of blind sac, the slit-like opening into which is the 

 blastopore. 



If at this time the egg of the hen could be observed 

 from the external surface over the germinal disc, one 



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.) 



would see an oval area undermined posteriorly. At the 

 centre of the posterior flap a little notch soon makes its 

 appearance, which becomes deeper and ends in a groove 

 extending anteriorly half the length of the disc the 



