Page:Proceedings of the Royal Society of London Vol 60.djvu/378

Rh kind, such as that of Professor Oscar Hertwig, who studied the various monstrosities obtained by mechanical compressions, by supermaturation of the ovum, and addition of various salts to water in which the eggs were developing. Similar work has been done upon sea-urchin’s eggs by several biologists (Pouchet and Chabiy, Herbst, &c.).

There are other most valuable records of the results obtained by separating the several spheres of the early stages of segmentation of eggs of Ctenophores, Echinoderms and Amphioxus by Chun, Driesch, Wilson, and others.

Kastschencko, by injuring portions of the germ ring of Elasmobranch embryos, has produced very valuable evidence in connexion with the concrescence theory, and Morgan has by similar methods examined the development of Teleosteans.

As far as I know, an experimental study of the development of the avian blastoderm has not hitherto been made.

The method adopted, which is very simple, was as follows. The egg was first of all opened at one side, and a bristle inserted into the yolk at some distance away from the blastoderm, to mark its anterior and posterior axis. The yolk, with its surrounding albumen, was then turned out into a glass vessel having a rather greater capacity than that of an ordinary egg shell. The yolk was arranged so that the blastoderm floated uppermost, and a wire or celluloid ring was placed over it to prevent the yolk from floating to the surface. A fine sable hair was then inserted in the blastoderm, and its position measured by a micrometer eye-piece and recorded in tenths of a millimetre. The vessel was filled up with albumen and covered with a glass lid, and placed in the incubator at a temperature of 104° P. Under these conditions, although development was slower than under normal conditions, many embryos reached, after about fortyeight hours, an age equivalent to a normal thirty to thirty-six hours’ chick with nine or ten pairs of mesoblastic somites. To come now to the results of the experiments, it is clear that if Duval’s theory is correct a hair inserted in the area opaca at the point a (fig. A (i)) ought to appear, in a specimen in which the primitive streak is formed, somewhere in front of the primitive streak. It, however, does not; it appears in the area opaca behind the primitive streak at a, fig. A (ii).

So again if the primitive streak is formed by the concrescence of the posterior margin, the sables inserted at the posterior edge at XX should either appear in the primitive streak or else prevent its formation.