The first of these conditions is only partially fulfilled, but, considering the rudimentary condition of the whole structure, no great stress can, it seems to me, be laid on this fact.
The second condition seems to me very completely satisfied. Where the two edges of the blastoderm become united we should expect to find a complete fusion of the layers such as takes place in the primitive streak; and the fact that in the primitive streak the hypoblast does not so distinctly coalesce with the mesoblast as the mesoblast with the epiblast cannot be urged as a serious argument against me.
The growth outwards of the mesoblast from the axis of the primitive streak is probably a remnant of the invagination of the hypoblast and mesoblast from the lip of the blastopore in Amphibia, &c.
The groove in the primitive streak may with great plausibility be regarded as the indication of a depression which would naturally be found along the line where the thickened edges of the blastoderm became united.
With reference to the third condition, I will make the following observations. The neurenteric canal, as it is placed at the extreme end of the embryo, must necessarily, with reference to the embryo, be the hindermost section of the blastopore, and therefore the part of the blastopore apparently behind this can only be so owing to the embryo not being folded off from the yolk sac; and as the yolk sac is in reality a specialised part of the ventral wall of the body, the yolk blastopore must also be situated on the ventral side of the embryo.
Kölliker and other distinguished embryologists have believed that the epiblast of the whole of the primitive streak became part of the neural plate. If this view were correct, which is accepted even by Rauber, the hypothesis I am attempting to establish would fall to the ground. I have, however, no doubt that these embryologists are mistaken. The very careful observations of Gasser shew that the part of the primitive streak adjoining the embryo becomes converted into the tail-swelling, and that the posterior part is folded in on the ventral side of the embryo, and, losing its characteristic structure, forms part of the ventral wall of the body. On this point my own observations confirm those of Gasser. In the lizard the early appearance of the neurenteric canal at the front end of the primitive streak clearly shews that here also the primitive streak can take no share in forming the neural plate.
The above considerations appear to me sufficient to establish my hypothesis with reference to the nature of the primitive streak, which has the merit of explaining, not only the structural peculiarities of the primitive streak, but also the otherwise inexplicable position of the embryo of the amniotic vertebrates in the centre of the blastoderm.
Complete List of Reference Letters.
am. Amnion. ch. Notochord. ch´. Notochordal thickening of hypoblast. ep. Epiblast. hy. Hypoblast. m.g. Medullary groove. me.p. Mesoblastic plate. ne. Neurenteric canal (blastopore). pr. Primitive streak.