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HAND-BOOK OF PHYSIOLOGY. 



the muscles and the cerebro-spinal and sympathetic nerves, with the vas- 

 cular and genito-urinary systems, and all the digestive canal with its 

 various appendages with the exception of the lining epithelium above 

 mentioned. 



FlKST KUDIMEKTS OF THE EMBRYO AKD ITS CHIEF 



Germinal Area. The position in which the embryo is about to appear 

 is early marked out by a central roundish opacity in the blastoderm, due 

 to the accumulation of cells in this region. This germinal area, which is 

 at first circular, changes its shape, becoming pyriform, and finally an 

 elongated oval constricted in the middle like a savoy biscuit. 



The central portion becomes transparent, 

 and thus we have an area pelludda, sur- 

 rounded by an area opaca (Fig. 409). 



Primitive Groove. The first trace of 

 the embryo is a shallow longitudinal groove 

 (primitive groove), which appears toward the 

 posterior part of the area pellucida (Figs. 

 409, 412). 



Medullary Groove. The primitive 

 groove is but transitory, and is soon dis- 

 placed by the medullary groove, which first ap- 

 pears at the anterior extremity of the future 

 embryo, and grows backward, gradually caus- 

 ing the disappearance of the primitive groove. 

 Laminae dorsales. The medullary canal is bounded by two longitu- 

 dinal elevations (lamina dorsales), which are folds consisting entirely of 

 cells of the epiblast: these grow up and arch over the medullary groove 



FIG. 409. Impregnated egg, with 

 commencement of formation of em- 

 bryo : showing the area germinativa 

 or embryonic spot, the area pellu- 

 cida, and the primitive groove or 

 trace (Dalton), 



FIG. 410.-Transverse section through embryo chick (26 hours), a, epiblast; ft, mesoblast; c, hy- 

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

 dorsal ridge (Klein). 



(Fig. 411) till they coalesce in the middle line, converting it from an open 

 furrow into a closed tube the primitive cerebro-spinal axis. Over this 

 closed tube, the walls of which consist of more or less cylindrical cells, the 

 superficial layer of the epiblast is now continued as a distinct membrane. 



