586 ON THE EARLY STAGES IN THE DEVELOPMENT OF THE EMU, 



oblique cleft. There is no trace of any such break in any of my 

 series of sections ; the axial plate in fact is completely continuous 

 with the head-process. It is very difficult, however, to say 

 whether or not the thickening constituting the ' head-process ' is 

 brought about by an invasion of cells from the primitive streak. 

 The former (which is more correctly described as medtdlary plate 

 of lower layer) is continuous with the latter by a process of cells, 

 but whether cells travel forwards through this process and add to 

 the thickness is hardly capable of being decided. It seems proba- 

 ble that the ' head-process ' is merely the continuation forwards 

 for a short distance of that axial thickening of the lower layer, 

 which, as above described, accompanies the formation of the 

 primitive streak, and, except that it does not coalesce with the 

 epiblast, the history of the lower layer is the same here as further 

 back ; a layer of flattened hypoblast is derived from the lowest of 

 its cells, and the rest is converted into mesoblast. 



It may be useful to sum up here the history of the formation of 

 the mesoblast in the emu. When the primitive streak is first 

 formed there are only two layers in the blastoderm. These two 

 layers — upper and lower — both become thickened along the axial 

 line of the area pellucida in its posterior prolongation, and there 

 coalesce — the coalescence f>^t^s t^^e thickenings constituting the 

 primitive streak. Thethickening of the lower layer extends forwards 

 a short distance in front of the anterior end of the primitive streak 

 to constitute the 'head-process.' The lowermost cells of the 

 lower laver about the time of the first appearance of the primitive 

 streak begin to be differentiated into a series of flattened cells 

 which afterwards unite to form a continuous layer of cells — the 

 hypoblast. This becomes a complete layer much later in the 

 region of the ' head-process ' than in the region of the primitive 

 streak. In the lateral parts of the area pellucida, where the 

 lower layer is thin, its cells become entirely converted into the 

 single layer of hypoblast cells. In the middle the cells which 

 remain after the hypoblast has become formed go to form the 

 earliest rudiment of the mesoblast ; the hypoblast becomes 

 separated from this rudimentary mesoblast, and the latter from 



