Hi EXPLANATION OF PLATES 44, 45. 



SERIES C, i and 2. Sections through a blastoderm with a young primitive streak. 

 i. Section through the front end of the primitive streak. 2. Section through 

 the primitive streak, somewhat behind i. Both figures shew very clearly the differ- 

 ence in character between the cells of the epiblastic mesoblast of the primitive streak, 

 and the more granular cells of the mesoblast derived from the hypoblast. 



FIG. D. Longitudinal section through the axial line of the primitive streak, and 

 the part of the blastoderm in front of it, of an embryo duck with a well-developed 

 primitive streak. 



PLATE 44. 



SERIES E, i, 2, 3 and 4. Sections through blastoderm with a primitive streak, 

 towards the end of the first stage. 



i. Section through the anterior part of the area pellucida. 2. Section a little 

 way behind i shewing a forward growth of mesoblast from the primitive streak. 3. 

 Section through primitive streak. 4. Section through posterior part of primitive 

 streak, shewing the great widening of primitive streak behind. 



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 2. 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. 



