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 



I 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 



