Page:Proceedings of the Royal Society of London Vol 60.djvu/380

Rh margin of the area pellucida. The area opaca takes no part at all in the formation thereof.

I may add that from a careful examination of surface views of living and preserved specimens, and from sections, I find it just as difficult to corroborate Duval's account of the formation of the primitive streak as I do from the experimental study I have just

I come now to the second part of the inquiry ; namely, what part of the actual embryo does the primitive streak give rise to ?

A sable hair inserted at the centre of the blastoderm appears at t e anterior end of the primitive streak.

If such a specimen is allowed to develop for some hours longer, until the medullary plate and medullary groove are clearly formed, these structures are found to be in front of the sable hair; that is to. sav the sable hair is still at the front end of the primitive streak ffio- B) If a specimen, in which the sable hair has been inserted at the°'same sp o t-that is to say, at the centre of the unincubated blastoderm—is left until several pairs of mesoblastic somites have appeared, the hair is found at the level of the most anterior pair of

From these specimens it seems clear that all those parts in front of the first pair of mesoblastic somites (that is to say, the heart, the brain, and medulla oblongata, the olfactory, optic and auditory

Fig. B.—(i) Diagram of unincubated Blastoderm. (ii) Blastoderm after 24 hours’ incubation. (iii) Blastoderm after 40 hours’ incubation. AO, area opaca; AY, area vasculosa; NG-, medullary groove; PS, primitive streak $ X, point of insertion of sable hair.