THE PRIMITIVE STREAK OF THE RABBIT. 195 



number of cells seem to be lying separate or forming a loose 

 network between the epiblast and hypoblast. These are meso- 

 blast cells. 



As the sections pass more posteriorly this primitive streak 

 knob becomes smaller and ends rather abruptly, as seen in 

 the sagittal section, fig. 17. But forwards the thickening 

 of the epiblast is continued much further, getting less very 

 slowly, as figs. 14 and 15 show, which are taken along the lines 

 14 and 15 in fig. 4. This narrow band of thickened epi- 

 blast is seen as the irregular lower surface of the epiblast in 

 fig. 17. It extends very nearly halfway, along the length of 

 the embryonal area. 



I think it is pretty clear that this is part of the same area 

 of proliferation as that through which fig. 16 passes, as will 

 appear more certainly later on. Hence the most anterior 

 point to which this reaches is a point of very great importance. 

 This is marked A. PS. in figs. 3 and 4. 



The proliferation of cells is so rapid at the posterior end as 

 to cause an eminence in transverse section (fig. 16 ; evident 

 also in fig. 17). The three sections, figs. 14, 15, 16, diff'er 

 only in amount of proliferating area and accumulation of 

 cells proliferated. This I take to mean a difiference only in 

 intensity. Excepting in intensity there is no difference sug- 

 gested by the structure of the secondary area of activity in the 

 three sections. 



The secondary area of proliferation is now fully established, 

 and has changed in shape from an ill-defined spot in fig. 2 to 

 a more and more elongated area in figs. 3 and 4. 



In fig. 5 it has still further lengthened. Figs. 21, 22, 23 

 are sections taken across this specimen at corresponding spots 

 to figs. 14, 15, 16 respectively, and must be compared one 

 with the other. It may be noticed that there is here ex- 

 tremely little difference in the extension transversely of the 

 area of proliferation. The cells, which appear to be entirely 

 separated, extend further from the middle line on each side, 

 but the area of fusion itself (between epiblast and mesoblast) 

 differs very little. This is especially noticeable in the most 



