III.] THE PRIMITIVE GROOVE. 47 



running along its axis. In fresh specimens viewed with 

 transmitted light, this furrow appears as a linear trans- 

 parency, but in hardened specimens seen under reflected 

 light may be distinctly recognized as a furrow or narrow 

 groove, the bottom of which being thinner than the sides 

 appears more transparent when viewed with transmitted 

 light. It is known as the primitive groove. The nature of 

 the changes by which it is brought about can only be learnt 

 by the study of vertical sections (Fig. 10). These teach us 

 that the opacity which marks out the primitive streak is 

 chiefly due to a thickening of the mesoblast. In the for- 

 mation, however, both of the primitive streak, and especially 

 of the primitive groove, the epiblast also plays an important 

 part. 



During these twelve hours the epiblast has been spreading 

 rapidly, much more rapidly than the other two layers. Over 

 the white yolk in the region of the opaque area it forms a 

 layer one cell deep, but at the same time has become two or 

 three cells deep in the centre of the pellucid area. In the 

 pellucid area its constituent cells have become narrower (6/^) 

 and more columnar, but over the opaque area flatter and 

 broader (12 //,) than they were at first. At the 12th hour 

 therefore we find a distinct histological difference between 

 the epiblast cells of the pellucid and those of the opaque 

 area. 



Over the thickening of the mesoblast, which forms the 

 basis of the primitive streak, the epiblast is also thickened ; 

 the hypoblast, however, remains here, as in the rest of the 

 blastoderm, a flat sheet consisting of a single layer of flat- 

 tened (seen in sections as a single row (Fig. 10, G) of spindle- 

 shaped) cells, which become larger ^nd more irregular at the 

 periphery. The thickening of the mesoblast and epiblast 

 in the region of the primitive streak causes the upper 

 outline of the blastoderm as seen in sections to rise above 

 the general surface in a gentle curve (Fig. 10). 



The primitive groove is formed almost entirely by a 

 pushing in or depression of the epiblast at the summit of 

 this curve. 



The thickness of the epiblast remains about the same on the sides as at the 

 bottom of the groove. The mesoblast, on the contrary, is thinner immediately 

 beneath the bottom of the groove than at the two sides, where it is decidedly 



