EXPLANATION OF PLATES XIII, XIV & XV— continued. 



PLATE XIV. 



Series E, 1 and 2. — Sections through a blastoderm with primitive groove. 

 1. Section showing a deep pit in front of primitive streak, probably 

 an early indication of the neurenteric canal. 2. Section immediately 

 following 1. 



Fig. G. — Section through blastoderm with well developed primitive streak, 

 showing an exceptionally deep slit-like primitive groove. 



Series H, 1 and 2. — Sections through a blastoderm with a fully-developed 

 primitive streak. 



1. Section through the anterior part of area pellucida, showing the 

 cubical granular hypoblast cells in this region. 2. Section slightly 

 behind 1, showing the primitive hypoblast cells differentiated into stellate 

 cells, which can hardly be resolved in the middle line into hypoblast 

 and mesoblast. 



Series I, 1, 2, 3, 4 and 5.— Sections through blastoderm somewhat older 

 than Series H. 



1. 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. 



PLATE XV. 



Series K, 1, 2, 3, 4 and 5. — Sections through a blastoderni in which the 

 first trace of uotochord and medullary groove have made their appear- 

 ance. Rather more than half the section is represented in each figure, 

 but the right half is represented in 1 and 3, and the left in 2 and 4. 



1. Section through notochord immediately behind the head-fold. 

 2. Section showing medullary groove a little behind 1. 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. 



