744 GENERATION AND DEVELOPMENT. 



" area germinativa" or germinal area, is the part at which 

 the embryo first appears. 



At first the area germinativa has a rounded form, but 

 Fig. 209.* it soon loses this and becomes 



oval, then pear-shaped, and 

 while this change in form is 

 taking place, there gradually 

 appears in its centre a clear 

 space or area pellucida (fig. 

 209), bounded externally by 

 a more opaque circle, the 

 obscurity being due to the 

 greater accumulation of nu- 

 cleated cells and nuclei at 

 that part than in the area pellucida. 



The first trace of the embryo in the centre of the area 

 pellucida consists of a shallow groove or channel, the 

 primitive groove (fig. 209), formed of the external or 

 serous fold of the germinal membrane, the groove being 

 wider at its anterior or cephalic extremity, and tapering 

 towards the opposite extremity. 



Coincidently with the formation of the primitive groove, 

 two oval masses of cells, the lamina dorsales, appear, one 

 on each side of the groove. At first scarcely elevated above 

 the plane of the germinal membrane, they soon rise into 

 two prominent masses, the upper borders of which gradually 

 tend towards each other, turning inwards over the primi- 

 tive groove. The parts from opposite sides then unite, 

 and convert the primitive groove into a tube, large and 

 rounded in front, narrow and lancet-shaped behind, which 

 is the central canal of the cerebro-spinal axis, and contains 

 the rudimental spinal cord and brain, which are developed 

 in its interior (fig. 210). 



* Fig. 209. (After Dalton.) Impregnated egg, with commencement 

 of formation of embryo ; showing the area germination or embryonic 

 spot, the area pellucida, and the primitive groove or trace. 



