THE PRIMITIVE STREAK OP THE RABBIT. 193 



upon, or just within the posterior border of the previously cir- 

 cular embryonic disc (see PS., PI. 20, fig. 2). 



Whether this activity concerns in the first instance both the 

 cells derived from the outer layer of epiblast and those 

 derived from the inner layer of epiblast equally, I cannot form 

 at present a definite opinion. 



Very soon the centre of this ragged area becomes thickened, 

 and the embryonal area has now the appearance shown in fig. 3. 

 The original outline of the embryonic disc is fairly distinct, but 

 of course slightly larger than in the earlier stage (fig. 1). This 

 thickening, which is due to a greater accumulation of cells at 

 this point, would seem to be the result of a greater activity of 

 cell multiplication at this spot. 



As to the cause of this sudden increase of activity I have no 

 suggestion to make. It is, of course, quite possible that the 

 activity was present from the first, and that it is only owing to 

 changed conditions that it now becomes evident as a heaping- 

 up of cells. To give a strictly epigeuetic explanation some such 

 cause ought to be adduced, but for the present I am not able 

 to suggest any, and so must ascribe it to a palingenetic cause. 



It is not possible to point out the exact boundaries of this 

 area of increased activity, but probably if that portion of the 

 epiblast which is distinctly thickened is taken to be the area of 

 increased activity, I think the error will not be great. Up 

 to now, and for some considerable time to come, there is no 

 change noticeable in the condition of the hypoblast. 



The darkened area PS. in fig. 3 may be taken as the area 

 of increased activity. The posterior border of this area is 

 semicircular, the anterior border is conical. The darkest part 

 of this area is about the position of the posterior border of the 

 original embryonic disc. 



Fig. 4 is a slightly later stage. The anterior portion of the 

 embryonal area still shows a circular darker part, which I take 

 to correspond approximately to the original circular embryonic 

 disc of the sixth day. The secondary area of activity is of the 

 same nature as in the preceding figure, but its anterior conical 

 part is slightly more pronounced. On either side of it there are 



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