Development of the First Day 137 



ment of the lower layer cells into a thin but 

 definite layer. 



2 The primitive streak, with the primitive 

 groove along its axis, is formed as a linear 

 proliferation of cells from the lower side of the 

 ectoblast. These cells spread out on each side 

 to form a part of the mesoblast. 



3. The pellucid area becomes pear-shaped ; 

 and the vascular area becomes distinguishable 

 as an inner zone of the area opaca. 



4. The medullary plate is formed in front 

 of the primitive streak, and its edges become 

 elevated, as the medullary folds, and meet 

 (without yet fusing), in the region of the brain, 

 to form the medullary canal. 



5. The notochord is formed as a medial, 

 rod-shaped thickening of the entoblast, in 

 front of the primitive streak. 



6. The development of the head-fold marks 

 the beginning of the head. 



7. The mesoblast becomes split into two 

 layers, the somatopleure and the splanchno- 

 pleure, with the body-cavity between them. 



8. The vertebral plates are separated from 

 the lateral plates, and in the former about five 

 pairs of mesoblastic somites are formed. 



It may be well, before taking up the de- 



