DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 129 



and Reptiles in other points, so there also is in the existence of a 

 neurenteric canal. At a rather early stage there is already noticeable, 

 at the anterior end of the primitive streak, a small spot, at which, 

 in consequence of cell-proliferation, a large amount of material is 

 accumulated. It is known under the name of HENSEN'S node (fig 90 

 hk). This is important chiefly because a narrow canal, the canalis 

 neurenterieus (en), passes through it, and leads from the outside into 

 the interior of the blastodermic vesicle. The presence of this canal 

 has already been established by several investigators by VAN 

 BENEDEN in the Eabbit and the Bat, by BONNET in the Sheep, by 

 HEAPE in the Mole, and by GRAF SPEE in a young human embryo. 

 The latter exhibited a still widely open medullary furrow. At the 

 beginning of the primitive groove there was a wide, roundish, 

 triangular orifice, which traversed the germ-disc, and was surrounded 

 by a ring-like elevation corresponding in position to HENSEN'S node. 



I have dwelt upon the primitive streak more at length, and have 

 considered more in detail its first appearance and its topographic 

 relations to other organs, because from a developmental standpoint 

 it is a very important structure, and one the significance of which 

 is still much discussed. For it corresponds to the blastopore of the 

 lower Vertebrates, and is important as the region from which the 

 middle germ-layer takes its origin. While I postpone an exposition 

 of the grounds which warrant us in designating the primitive groove 

 as blastopore, I shall at once consider the development of the middle 

 germ-layer. Information concerning this is to be got from cross 

 sections, which should be made, as in the Amphibians, (1) in front 

 of the primitive groove, (2) in the region of the groove, and (3) back 

 of it, both in younger and older embryos. 



In embryonic fundaments which have reached the stages repre- 

 sented in figs. 81 B, 85, and 89, the middle germ-layer is already 

 begun in the immediate vicinity of the primitive groove, and causes 

 the opacity which appears upon both sides and in front of it. Cross 

 sections through the cephalic process of the primitive streak now 

 allow the establishment of a complete agreement in one fundamental 

 point between Amphioxus and the Amphibia on the one hand, and 

 Selachians, Reptiles, Birds, and Mammals on the other. 



Along a narrow median streak, in the former groups in front of the 

 blastopore, in the latter in front of the primitive groove, the embryonic 

 fundament is composed of only two germ-layers, of which the lower is 

 destined to become the chorda. At both sides of these regions the two- 

 layered condition passes abruptly in all Vertebrates into a three-layered 



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