148 GENERAL BIOLOGY. 



Study (a) and (6) ; then begin at the posterior 

 end (why?) of (c), and going toward the 

 anterior find the regions of the primitive 

 streak and medullary folds. Observe in 

 the medullary-fold region the notochord 

 and the mesoblast. In which region are 

 the medullary folds most nearly closed ? 

 After studying (c), sketch (a) and (b) with AA 

 2, after having examined them with D 2. 

 Sketch the outlines of the different germ- 

 layers with a black pencil and fill in the 

 space between with faint color, using al- 

 ways the same color for the same germ- 

 layer. 

 IV. The Late First Day. 



1. Surface. Sketch with AA 2. 



2. Sections. 



Begin at the posterior end of the series. 

 Observe : 



(a) The primitive streak. 



(b) The medullary folds. 



(c) The mesoblast. 



Somatic layer. 

 Splanchnic layer. 

 Mesoblastic somites. 



(d) The notochord. 



(e) The closure of the medullary folds to form 



the neural canal. 

 (/} The closure of the archenteron. 



