183 F. M. BALFOUR AND V. DEIGHTON. 



elongation of the primitive streak appears to be that adopted by 

 Gerlach. 



Our next stage includes roughly the period commencing 

 slightly before the first formation of a groove along the primi- 

 tive streak, known as the primitive groove, and terminating 

 immediately before the first trace of the notochord makes its 

 appearance. After the close of the last stage the primitive 

 streak gradually elongates, till it occupies fully two thirds of the 

 diameter of the area pellucida. The latter structure also soon 

 changes its form from a circular to an oval, and finally becomes 

 pyriform with the narrow end behind, while the primitive streak 

 occupying two thirds of its long axis becomes in most instances 

 marked by a light linear band along the centre, which constitutes 

 the primitive groove. 



In surface views the primitive streak often appears to stop 

 short of the hinder border of the area pellucida. 



During the period in which the external changes, which we 

 have thus briefly described, take place in the area pellucida, 

 great modifications are effected in the characters of the germinal 

 layers. The most important of these concern the region in 

 front of the primitive streak ; but they will be better understood 

 if we commence our description with the changes in the primitive 

 streak it-self. 



In the older embryos belonging to our last stage we pointed 

 out that the mesoblast of the primitive streal: was commencing 

 to extend outwards from the median line in the form of two 

 lateral sheets. This growth of the mesoblast is continued rapidly 

 during the present stage, so that during the latter part of it any 

 section through the primitive streak has approximately the cha- 

 racters of Ser. I, 5. 



The mesoblast is attached in the median line to the epiblast. 

 Laterally it extends outwards to the edge of the area pellucida, 

 and in older embryos may even form a thickening beyond the edge 

 (fig. g) . Beneath the denser part of the mesoblast, and attached 

 to the epiblast, a portion composed of stellate cells may in the 

 majority of instances be recognised, especially in the front part 

 of the primitive streak. We believe these stellate cells to be in 

 the main directly derived from the more granular cells of the 

 previous stage. The hypoblast forms a sheet of flattened cells, 

 which can be distinctly traced for the whole breadth of the area 

 pellucida, though closely attached to the mesoblast above. 



In sections we find that the primitive streak extends back to 

 the border of the area pellucida, and even for some distance 

 beyond. The attachment to the epiblast is wider behind; but 

 the thickness of the mesoblast is not usually greater in the 

 median line than it is laterally, and for this reason probably the 



