352 Mr. R. Assheton. An Experimental Examination 



(i) A 



r. A. (i) Diagram of the unincubated Blastoderm of a Bird, (ii) Diagram of 

 the Blastoderm after the complete Formation of the Primitive Streak. 



On the contrary, they are found far behind the primitive streak in 

 the area opaca. 



These facts seem to show that the primitive streak is not formed 

 from the posterior edge of the blastoderm as Duval maintains. 



As a rule, in the unincubated blastoderm the area opaca and area 

 pellucida are very fairly well defined. 



If, when this is the case, a sable hair is inserted jusb within 

 the area pellucida at the point b, or if, when there is no such 

 distinction, the sable is inserted about one quarter the distance from 

 P to A, the sable hair is found, after the development of the primi- 

 tive streak, piercing the posterior end of the primitive streak 

 whereas, according to Duval's account, it ought to be somewhere in 

 front of the primitive streak. 



If a hair is inserted in the median line rather further towards the 

 centre of the blastoderm, it is found near the middle of the primitive 

 streak, or, if placed about half way between the inner edge of the 

 posterior part of the area opaca and the centre of the blastoderm (as 

 at c), it is found in the anterior part of the primitive streak ; and, 

 when the sable is inserted at the centre of the blastoderm, it appears 

 at the front end, or just in front of the primitive streak (fig. A, d). 



The foregoing proves, I think, conclusively, that the primitr 

 streak is developed from that portion of the unincubated blastoden 

 which lies between the centre of the blastoderm and the posi 



