I Jenkinsov, Germinal Layers of Vertebrates. 



It is clear that this portion of the notochord and 

 mesoderm is brought into position concomitantly with 

 the overgrowth of the blastoporic lip ; and that the two 

 sheets of mesoderm are confluent with one another 

 posteriorly in the similarly formed mesoderm behind the 

 ventral lip. The whole of this mesoderm is subequatorial 

 or, after rotation, postero-dorsal. Antero-ventrally the 

 mesoderm and the notochord are completed by a different 

 process ; as the yolk-cells are pushed into the segmenta- 

 tion cavity those which lie next the roof of the latter, next 

 the ectoderm that is, may be seen to be in a state of rapid 

 sub-division (yFig. 2, C — F : mes. 2){Fig, 3) A) ; a layer of 

 cells smaller than the yolk but larger than the ectoderm 

 cells is thus produced, placed on what will be the antero- 

 ventral side of the embryo (A';^. 3, B), and continuous with 

 the already described postero-dorsal mesoderm around 

 the equator. 



The germ layers are thus definitely established ; at 

 the lips of the blastopore all three are, of course, continuous 

 with one another. It only remains, for us to discuss a 

 httle more minutely their origin from the two sets of cells 

 — the so-called primary layers — observable in the &gg at 

 the end of segmentation. 



A very considerable portion of each germ-layer is 

 actually brought into being during the closure of the 

 blastopore. What is the origin of the cells which form 

 the fold by the overgrowth of which this closure is 

 effected ? Are they derived exclusively from the cells of 

 the animal hemisphere alone, as some (notably Lwofif) 

 have maintained ; or, while the outer ectodermal sheet is 

 to be traced to the animal cells, is the inner sheet merely 

 due to the invagination of yolk-cells as was first stated by 

 the author of the " Gastraea-theorie " ? As far as I can 

 see both these views are erroneous. The yolk-cells though 



