FOBMATION AND FATE OF THE PRIMITIVE STREAK. 101 



and mesoblast. It is altogether ventral to the archenteron, so that 

 there can be no fusion with true hypoblast. There is no fusion either 

 with the yolk-cells. The surface is groooved, as in the sections anterior 

 to the amis. 



The fusion of layers continues through six more sections ; that is to 

 say, there are about twenty sections ventral to the anus, in which 

 there is fusion of epiblast aucl mesoblast ; in other words, the anus is a 

 perforation through the primitive streak. 



Comparison of the " Primitive Streak '' of the Frog ivith the Primitive 

 Streak of the Chick. 



We have now described the history of the blastopore or anus of 

 Rusconi from the time of its first formation until it has been reduced 

 to a comparatively minute passage between the archenteron and 

 exterioi'. We have described this change as being due to the con- 

 crescence of the lower lips of the blastopore, as shown diagrammatically 

 in figs. 10, 18, 19, whereby there is produced a median streak charac- 

 terised by the fusion of layers, along the surface of which lies a groove, 

 stretching from the ventral lip of the remaining blastopore ventral- 

 wards as far as the original extension of the blastopore or anus of 

 Rusconi. To this structure and to the groove upon it we have applied 

 the terms primitive streak and primitive groove respectively. 



If one of our sections figured, e.g. fig. 15, is compared with the 

 figure of a transverse section of the primitive sti'eak of a chick on 

 p. 155 of Balfour's 'Comparative Embryology,' vol. ii (second edition), 

 the resemblance between the two sections is very marked. 



In each case there is an intimate fusion between epiblast and 

 mesoblast, and a more uncertain fusion between mesoblast and hypoblast. 

 In each case the surface is marked by a groove. 



Nor is it at all impossible that the loose pigmented cells noticed 

 above, and lettered ME" in figs. 14 and 15, may be compared with 

 Balfour's " layer of stellate cells " shown in the figure in the 

 ' Comparative Embryology ' referred to. In both cases they seem to 

 arise from the hypoblast rather than the epiblast. In the frog, however, 

 they are unrecognisable a short distance from the streak. 



In comparing the two streaks with regard to their relations to the 



