87 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



SERIES F, i and 2. Sections, through a blastoderm with primitive groove. 



i. Section shewing a deep pit in front of primitive streak, probably an early 

 indication of the neurenteric canal. 2. Section immediately following i. 



FIG. G. Section through blastoderm with well-developed primitive streak, shew- 

 ing an exceptionally deep slit-like primitive groove. 



SERIES H, i and i. Sections through a blastoderm with a fully-developed primi- 

 tive streak. 



i. Section through the anterior part of area pellucida, shewing the cubical 

 granular hypoblast cells in this region. 2. Section slightly behind i, shewing the 

 primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved 

 in the middle line into hypoblast and mesoblast. 



PLATE 45. 



SERIES I, i, 2, 3, 4 and 5. Sections through blastoderm somewhat older than 

 Series H. 



i. Section through area pellucida well in front of primitive streak. 2. Section 

 through area pellucida just in front of primitive streak. 3. Section through the front 

 end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4. 



SERIES K, 1,2, 3, 4 and 5. Sections through a blastoderm in which the first 

 traces of notochord and medullary groove have made their appearance. Rather more 

 than half the section is represented in each figure, but the right half is represented in 

 i and 3, and the left in 2 and 4. 



i. Section through notochord immediately behind the head-fold. 2. Section 

 shewing medullary groove a little behind i. 3. Section just in front of the primitive 

 streak. 4 and 5. Sections through the front end of the primitive streak. 



FIG. L. Surface view of blastoderm with a very young primitive streak. 



