BY WILLIAM A. HASWELL, M.A., D.SC. 583 



passes without interruption into the axial plate, and here a well- 

 formed hypoblast becomes first clearly marked off. In the region 

 in front of the primitive streak and behind the crescentic groove — 

 the region that is of the future medullary plate — the epiblast is 

 thicker than in the surrounding parts of the blastoderm, and its 

 cells have a more regular foi-m. The suture in the primitive streak 

 referred to nbove, has now altogether disappeared, and the two 

 halves are closely united throughout their length. 



A study of the three stages which have been described, renders 

 it evident that the primitive streak cannot grow forwards from 

 the posterior border of the area pellucida, as it is generally 

 described as doing ; but that it is formed from before backwards 

 simultaneously with an extension backwards in the form of a 

 narrow bay, of the area pellucida. The sub-germinal cavity, that 

 is to say, sends an axial bay backwards, the posterior part of the 

 germinal wall bends backwards at the same time along the border 

 of this bay, and there is thus formed a narrow posterior prolonga- 

 tion of the area pellucida, on the surface of which the primitive 

 streak appears. Its first rudiment is apparently an axial thickening 

 of the upper layer on the region of the area opaca which is to be 

 converted into this bay ; and as the bay extends back the lower 

 layer also thickens, the two thickenings uniting. The area pellucida 

 has meantime been extending itself by growth in all directions, 

 with the result that the anterior end of the primitive streak comes 

 to be situated not far behind the middle of the anterior circular 

 part of the area 2}dlucida. That there is, however, a certain 

 forward growth of the anterior end of the streak after it has 

 become formed, seems probable when we compare figures 1 and 2 

 in plate ; it is, however, of much less extent in the emu than in ' 

 the fowl. 



The accompanying woodcuts are designed to illustrate the history 

 of the formation of the primitive streak in the emu. Only a part 

 of this history is traceable in the ontogeny of the individual, and 

 much less than at the outset I had hoped to find, — little more in 

 fact than in the chick, save that the mode of growth of the 



