THE PRIMITIVE STREAK OP THE RABBIT. 199 



men. The most posterior^ fig. 34, is iu no way different from 

 fig. 27, except that the proliferating area may be slightly 

 broader. A similar remark may be offered with regard to the 

 most anterior section, fig. 33, namely, that except the area of pro- 

 liferation maybe slightly broader, it very closely resembles fig. 25. 



In both figs. 32 and 34 the mesoblastic plates on each side 

 of the fused area are thicker than in tigs. 25 and 27. 



The section that passes through the middle of the length of 

 the primitive streak presents the most noteworthy appearance. 

 In this, fig. 33, the breadth of the area of fusion is very much 

 greater than in any of the specimens previously described. 

 There is no groove ; in fact, there is, in the place of a groove, 

 actually an eminence (P. S.). The cells proliferated are much 

 more numerous and more crowded within a given space as 

 compared with fig. 31, and still more so as compared with fig. 26. 



The total length of this secondary area of proliferation is 

 now only about one third of what it was when at its maximum 

 length as in fig. 6. Thus it has apparently become shorter 

 and thicker. 



In an older stage, when there are as many as twelve proto- 

 vertebrae formed, the foundations of the most bulky portion of 

 the body have been laid down. The secondary area of pro- 

 liferation is now less conspicuous. Its length is not much 

 more than one fifth, if so much, of its greatest attained length. 



In stages much later than this it is not possible to observe 

 it in surface views, as such parts as remain are then placed at 

 the extreme tip of the tail, after the complete development of 

 which it disappears. 



To sum up — the secondary area of activity arises as a 

 small spot excentrically placed to the primary centre 

 of activity. It increases in magnitude, then becomes 

 elongated, aud very much reduced in breadth towards 

 tlie centre of its length, and is deeply grooved. 

 Hather suddenly, after attaining its greatest length, 

 the groove disappears entirely; the area becomes 

 much shortened, and thicker at the spot where 

 it had been so thin. Now, instead of a groove, there 



