THE DEVELOPMENT OP THE MOLE. 49 



end of the primitive streaky at a point relatively similar to that 

 drawn in section in fig. 34, the epiblast was involuted in the 

 middle line and a deep pit formed which opened below into 

 mesoblastj which is budded oif from the lips of the ingrowth. 

 At this point the epiblast, mesoblast, and hypoblast were united 

 in the middle line, but in front of it an axial rod of mesoblast 

 projected forwards for a short distance distinct from both epi- 

 blast and hypoblast, but soon becoming attached to, and indis- 

 tinguishable from, the hypoblast. In this condition it may be 

 spoken of as a thickened axial rod of hypoblast, and as such it 

 extends forwards for some sections, gradually becoming re- 

 duced in size and eventually giving place to the single row of 

 rounded hypoblast which elsewhere existed below the epiblast 

 in front of the primitive streak. 



The third section (fig. 35) demonstrates the structure of the 

 area throughout the remainder of the primitive streak in front 

 of the end knob. It is similar to fig. 34 except that (1) the 

 hypoblast forms a complete layer across the whole of the area, 

 and is nowhere combined with the layer of mesoblast ; (2) the 

 primitive groove is not present, and the number of epiblast cells 

 concerned in the formation of mesoblast is greater than before, 

 and (3) the mesoblast extends laterally, lying freely between 

 the epiblast and hypoblast, outside the limits of the area. 

 This is a typical section through the middle of the primitive 

 streak of all the specimens I have examined, and it may be 

 generally stated that throughout this region the epiblast only 

 gives rise to mesoblast. 



In the knob at the hind end of the primitive streak 

 (fig. 36) the three layers are again seen to be closely 

 combined, the hypoblast being indistinguishable from the 

 mesoblast, and the epiblast throughout nearly the whole 

 breadth of the area giving rise to mesoblast cells ; there is also 

 a much greater mass of the latter layer extending some dis- 

 tance beyond the limits of the area, which in this region is very 

 narrow. 



, The junction of hypoblast and mesoblast does not appear to 

 occcur in this region in all specimens, although the embryo 



