Page:Proceedings of the Royal Society of London Vol 60.djvu/382

Rh Fig. c.— (i) Diagram of unincubated Blastoderm. (ii) Blastoderm of Chick with seven pairs of Mesoblastic Somites.

that the primitive streak is converted directly into a part of the embryo, that is to say, the part of the embryo posterior to, and including the first pair of mesoblastic somites.

With regard to the area vasculosa, my experiments seem to indicate that the part of the blastoderm which becomes area vasculosa is that part which lies on the inner edge of the posterior part of the area opaca of the unincubated blastoderm. It is along this edge where, according to Roller,* a white crescent is always visible.

Roller, further, asserts that this white ci’escent is grooved. From this crescent and groove Roller derives rhe primitive streak and primitive groove by the conversion of the transverse crescent and groove into a longitudinal streak and groove.

I think that all recent authors are agreed that it is not grooved, and most admit that it has nothing to do with the primitive streak.

It is, however, quite true that a crescentic whiter area is sometimes visible here, but in, I think, the majority of cases there is nothing of the kind to be seen.

When it is present a sagittal section of the hinder part of the blastoderm seems to reveal its nature. In such a section a mass of inner-layer cells, which would, perhaps, be more properly described as a band of yolk containing numerous nuclei, although quite sharply marked off from the underlying yolk-mass, can be detected. This area corresponds in position to that part of the blastoderm from which, according to experiments made with bristles, the area vasculosa is derived (figs. D, E).

A sable hair inserted in the yolk beyond the limits of the blasto-

Akad. der Wissensch.,’ Wien, vol. 80, 1879. Untersucli. uber d. Bliifcterbildung im Huhnerkeim,” ‘ Arcb. f. mikr.-Anat.,’ vol. 20, 1881.
 * “ Beitriige zur Kenntniss des Hiihnerkeiras im Beginne der Bebriitung,” ‘ Sitz.