BY WILLIAM A, HASWELL, M.A., D.SC. 581 



very well-defined border, in front of which is a transverse dark 

 space with a convex anterior border and shading off behind into 

 the pi-imitive streak proper. 



In the anterior part of the ai'ea pellucida of this specimen, as 

 seen in sections, there are only two layers — epiblast and lower 

 layer. The cells of the latter have not here yet taken on their 

 flattened form, but are irregular and amoeboid, many of them 

 thickly loaded with granules, arranged in a single layer. The 

 epiblast consists in the middle of several layers of cells containing 

 in many instances large granules : at the sides it consists of a 

 single layer. The mesoblast has not yet extended into this 

 region. As we pass backwards the cells of the lower layer 

 gradually lose their amoeboid character and become more 

 flattened, though still irregular in shape — the change in their 

 form beginning in the middle line. 



The "head-process " (plate XII., fig. 10) is now larger than in 

 the preceding stage, and its cells have assumed an irregular, 

 sometimes stellate, form ; here and there, as before, there is a 

 flattened cell foreshadowing the hypoblast, but the majority of the 

 cells are manifestly assuming the form of stellate mesoblast cells. 

 Behind, as in the last stage, the head-process passes without 

 interruption into the axial plate. In the primitive streak itself 

 (plate XII., figs. 11 and 12) there is the usual axialiplate continuous 

 with the surface epiblast, its lateral wings extending outwards 

 between the ejnblast and the hypoblast, which latter has now in 

 this region become developed into a continuous layer of somewhat 

 flattened cells. The mesoblast extends outwards far beyond the 

 termination of the hypoblast in the germinal wall. 



In the hinder part of the primitive streak region (fig. 13) there 

 is below the primitive groove what appears like an imperfectly 

 united longitudinal cleft or suture in the axial plate. The hypo- 

 blast below this is continuous across the middle line, but in the 

 centre, just below the " suture," the ordinary hypoblast cells are 

 replaced by a large cell filled with coarse granules. Though this 

 is a fresh formation since the last stage, we have here an 



