392 



GASTRULATION 



b. The Gastrula in Relation to the Primitive Body Plan 

 If one watches a large transport plane preparing to take off at an airfield, 

 the following events may be observed: 



( 1 ) The cargo and passengers are boarded, the engines are warmed, and 

 the plane is taxied toward the runway. 



(2) Upon reaching the starting end of the runway, the engines are ac- 

 celerated, and the plane is turned around and headed in the direction 

 of the take-off. 



Fig. 188. Relationship between the presumptive organ-forming areas of the blastula 

 (diagram A) and the primitive tubular condition of the developing vertebrate body 

 (diagram C). The gastrula (diagram B) represents an intermediate stage. Consult chart 

 in text. 



