THE PRIMITIVE STREAK OP THE RABBIT. 211 



portion. So by adding to 1-38 mm. (length of fig. 5) the 

 sums of '02 mm. due to lengthening of the primitive streak 

 and 'le mm. the amount due to growth of anterior end, we 

 have '16 mm. still to account for to produce the length shown 

 in fig. 6. 



Where has this extra length been acquired ? In transverse 

 section it is evident that there is now a region which was not 

 to be found in the preceding stage. 



A section through this region is given in fig. 24. The most 

 noteworthy feature is the notochordal thickening. A few 

 sections further forward this thickening is quite absent. It is 

 surely legitimate to ascribe the appearance of this new region 

 as being the result of cells budded ofi" from the front end of 

 the primitive streak. This becomes much more evident in the 

 later stage (fig. 8). In this the total length of the embryonal 

 area is about 3*11 mm. 



Fig. 6 measured 1*72 mm. There has therefore been an 

 increase of 1*39 mm. The proliferating area measured the 

 same as in fig. 6, namely 1*2 mm. There is at the anterior 

 part of the embryonal area a very well marked circular area. 

 The neural groove along this area is narrow, while the 

 neural groove along the embryonal area between the above- 

 mentioned circular area and the anterior end of the primitive 

 streak is broad and shallow. The junction between the two 

 corresponds almost exactly with the circumference of the 

 circular area at the point Z. 



Does this anterior circular patch represent either exactly or 

 approximately the area of influence of the primary centre of 

 activity ? I wish to point out the possibility of all that part 

 between the anterior end of the primitive streak and the 

 posterior border of the circular anterior area (namely that 

 part of the embryonal area which is marked by the shallow, 

 broad, neural groove) having been formed entirely by the 

 activity emanating from the primitive streak area, or, to be more 

 exact, the anterior and antero- lateral parts of that area 

 of proliferation ; the parts of the embryonal area in front of this 

 region having been formed chiefly as the result of the activity 



