590 DEVELOPMENT OF THE IMPREGNATED EGG. 



The first sign of advancing organization in the external layer of 

 the blastodermic membrane shows itself in a thickening and con- 

 densation of its structure. This thickened portion has the form of an 

 elongated oval-shaped spot, termed the " embryonic spot" (Fig. 195), 



the wide edges of which are somewhat 

 Fi g- 195 - more opaque than the rest of the blasto- 



dermic membrane. Inclosed within 

 these opaque edges is a narrower color- 

 less and transparent space, the "area 

 pellucida," and in its centre is a delicate 

 line, or furrow, running longitudinally 

 from front to rear, which is called the 

 " primitive trace." 



On each side of the primitive trace. 

 Boa, *uhTT in the area pellucida, the substance of 

 mencement of formation of embryo: the blastodermic membrane rises up in 



showing embryonic spot, area pellu- . -, 



cida, and primitive truce. such a manner as to form two nearly 



parallel vertical plates or ridges, which 



approach each other over the dorsal aspect of the foetus and are 

 therefore called the '''dorsal plates." They at last meet on the 

 median line, so as to inclose the furrow above described and con- 

 vert it into a canal. This afterward becomes the spinal canal, and 

 in its cavity is formed the spinal cord, by a deposit of nervous 

 matter upon its internal surface. At the anterior extremity of this 

 canal, its cavity is large and rounded, to accommodate the brain 

 and medulla oblongata ; at its posterior extremity it is narrow and 

 pointed, and contains the extremity of the spinal cord. 



In a transverse section of the egg at this stage (Fig. 196), the 

 dorsal plates may be seen approaching each other above, on each 

 side of the primitive furrow or "trace." At a more advanced 

 period (Fig. 197) they may be seen fairly united with each other, 

 so as to inclose the cavity of the spinal canal. At the same time, 

 the edges of the thickened portion of the blastodermic membrane 

 grow outward and downward, so as to spread out more and more 

 over the lateral portions of the vitelline mass. These are called 

 the " abdominal plates ;" and as they increase in extent they tend 

 to unite with each other below and inclose the abdominal cavity, 

 just as the dorsal plates unite above, and inclose the spinal canal. 

 At last the abdominal plates actually do unite with each other on 

 the median line (at i, Fig. 197), embracing of course the whole 

 internal layer of the blastodermic membrane (5), which incloses in 



