Page:Proceedings of the Royal Society of London Vol 60.djvu/379

352 Mr. R. Assheton. An Experimental Examination Fig-. A.— (i) Diagram of the unincubated Blastoderm of a Bird, (ii) Diagram of the Blastoderm after the complete Formation of the PrimitiYe 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 just 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 primitive 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 primitive streak is developed from that portion of the unincubated blastoderm which lies between the centre of the blastoderm and the posterior