Page:Proceedings of the Royal Society of London Vol 60.djvu/381

354 organs and fore gat) are developed from that portion of the unincubated blastoderm which lies anterior to the centre of the blastoderm and that all the rest of the embryo is formed by the activity of the primitive streak area.

I have fonnd it very difficult to determine, exactly, the anterior limits of the embryo in the unincubated blastoderm. This, no doubt, is due to the fact that, for the production of the anterior end of the embryo, very complicated foldings of the blastoderm are called into play, and the insertion of a bristle or the infliction of any injury to the delicate parts of the blastoderm involved in the process, almost entirely prevents anything like a normal course of development.

However, such little success as I have had gives the following results:—A hair inserted at the most anterior border of the area pellucida is found far in front of the primitive streak,

A hair inserted only slightly in front of the centre of the blastoderm appears (in a specimen in which the medullary folds are just becoming visible) in the medullary plate in front of the primitive streak. In older specimens, after the head-fold has been formed, the embryos are extremely abnormal when the sable has been inserted in the region under discussion.

Indeed, very few will develop as far as the formation of the headfold.

The only facts I can derive,from the insertion of sable hairs in this area are

(1) That it interferes very seriously with the course of development. r

(2) That the bristle appears inside the two anterior horns of the area vasculosa.

(3) That if placed some little way anterior to the centre it is found apparently in front of the embryo, but it interferes so greatly with the head-fold that it is difficult to say whether it has, or has not, perforated the anterior part of the embryo.

I have shown that a hair inserted between the centre of the blastoderm and the hinder margin of the area pellucida is found after about twenty hours of incubation in the primitive streak. When a specimen in which the sable has been similarly placed is allowed to develop until several mesoblastic somites have been formed, it is found to be posterior to the first formed mesoblastic somites.

For instance, in the specimen with me, the blastoderm measured 4‘3 mm. in diameter. The sable was inserted 1*3 mm. from the posterior edge of the blastoderm. After forty-one hours of incubation seven pairs of mesoblastic somites had been formed, and the sable hair was a short distance posterior to the 7th pair of somites.

From such specimens as these we are, I think, bound to conclude