196 EICHARD ASSHETON. 



posterior of the sections, namely, figs. 16 and 23. The actual 

 area of proliferation is about the same. 



In the anterior sections, if there is a difference, it is that in 

 the more advanced specimen (fig. 5) the sections show a smaller 

 extent of proliferating area than in the younger specimen. 



In specimen fig. 5 there is now a very slight groove along 

 the surface of this area, so slight as to be scarcely recognisable 

 in sections fig. 22 (P. GR.). 



There is another point of interest. Up to this moment the 

 extreme anterior end of the secondary area of proliferation has 

 almost shaded away imperceptibly into the epiblast of the future 

 neural plate, at any rate it has always been the most slender 

 part of the structure. Now it has become a very well-marked 

 spot — it forms almost a knob — and is henceforth recognisable 

 as what other authors have called '' Hensen^s node." Fig. 21 

 passes through the centre of this. 



Fig. 6 is a later stage still. In this the secondary area of 

 proliferation has attained about its maximum length, and at 

 this moment shows the greatest degree of distinction into 

 different parts : (i) hindermost, a broad area of proliferating 

 surface, its posterior border forming an arc of a circle, 

 while anteriorly it tapers off into (ii) the middle region, a 

 long narrow strip of proliferating area deeply grooved along 

 its length ; more deeply grooved anteriorly and less posteriorly 

 (fig. 26). This again passes abruptly into (iii) the most 

 anterior part of the primitive streak or Hensen's node, a very 

 much thickened and compressed region of short extent (vide 

 section, fig. 25). 



This stage is the height of development of the primitive 

 streak. The secondary area of proliferation is at this stage a 

 perfectly typical primitive streak, with well-developed primitive 

 groove. 



In the preceding figure (fig. 5) the groove is very shallow 

 and wide, and easily recognisable in the whole specimen. In 

 fig. 6 the groove is very deep and narrow, and is only with 

 difficulty seen in surface views as its walls are nearly approxi- 

 mated, and it appears when seen by transmitted light as a 



