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HANDBOOK OF PHYSIOLOGY. 



In the posterior part of the area pellucida now appears an opaque 

 streak which extends about a third of the diameter of the area towards 

 the middle line. This is the Primitive streak. It is found on trans- 

 verse section of the blastoderm in this neighborhood to be due to a pro- 

 liferation downwards of cells, two or more deep, from the epiblast. The 



FIG. 444. Impregnated egg, with commencement of formation of embryo; showing the area 

 germinativa or embryonic spot, the area pellucida, and the primitive groove or trace (Dalton). 



area pellucida now becomes oval. As the primitive streak becomes more 

 defined, the area pellucida changes its oval for a pear shape, but the 



FIG. 445. Transverse section through embryo chick (26 hours), a, epiblast; 6. mesoblast; c, 

 hypoblast; d, central portion of mesoblast, which is here fused with epiblast; e, primitive groove; 

 /, dorsal ridge (Klein). 



streak increases in size faster than the area, and so after a time is about 

 two- thirds of its length. In the axis of the primitive streak a groove, 



m oa 



d ch, 



FIG. 446. Diagram of transverse section through an embryo before the closing-in of the medul- 

 lary groove, m, cells of epiblast lining the medullary groove which will form the spinal cord; /i, 

 epiblast; d, hypoblast; c/i, noto chord; u, protovertebra; sp, mesoblast; w, edge of lamina dorsalis, 

 folding over medullary groove (Kolliker). 



the primitive groove, runs. From the primitive streak the cells from 

 the under-surface of the epiblast now extend as lateral wings to the edge 



