DKVBLOPMBXT >F mi: -n\o UIMM.I: QERM LAYERS. 



ill other points, so there also is in the exigence of a 

 i it-u iv! it eric canal. At :i ratlin 1 early stage there is already not ic.-aMe, 



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, [t is known under the name ol llr-. ! dig 90 



M). Thi- i- important chiefly h rau>e a narrow canal, tlio eanalis 

 heurent'Tieus (en), passes through it. ami lead- from the outside into 

 t In- interior of the blastodermic vesicle. Tin' presence of this canal 



has already heen established by several in\v-t igaior. bv VAN 



HKXEDEN in the Rabbit and the Bat, by BONNKT in the Sheep, by 

 MKAPE in the Mole, and by GRAF SIM-IK 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, roundi>h, 

 triangular orifice, which travrr>'d the ireriii-<li>c. and was >urruhd"il 

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



I have dwelt upon the primitive streak more at length, and 1 

 considered more in detail its first appearance and its topographic 

 rel at ions to other organs, because from a d> velopmental standpoint 

 ii i- a very important structure, and one the significance of which 

 i> still much discussed. For it correspond > to the blast opore of the 

 lower AVrtel>rates. and is important as the region from which the 

 middle i^erm-layer takes its origin. While I postpone an exposition 

 of the grounds which warrant us in designating the primitive groove 

 as blast opore, I shall at once consider the development of the middle 

 ::! m-layer. Information concerning this is to be got from cross 

 us, 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- 

 s'nted in figs. 81 B, 85, and 89, the middle g rm -lay. r is already 

 liegiin in the immediate vicinity of the primitive groove, and cau-e- 

 the opacity which appears upon both >ide> 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. Uirds. and Mammals on the other. 



J/ / the 



Ihishqnm-, in th? luttvr in front ofthejn''i, 

 n.i li'hunent is composed of only A V, / n-/,fr/t tltn l>m-er is 



'ncd to become the clofl'i. At /<//, .v //-. (//' t1 t >s>' r^jnnts t/^ \ 

 layered coml.iti<.m juttse* <(!>r},tl ;i if*:s into a three-layered 



9 



