THE PRIMITIVE STREAK OE THE RABBIT. 217 



bring about the compression of the primitive steak, without 

 assuming a conversion of it in situ, as it were, into the 

 embryo. 



On Ph 22 will be found six diagrams which illustrate my con- 

 ception of the lines of growth between the stages of figs. 1 and 

 8 of PI. 20. 



Fig. 40 represents the embryonic pole of the blastodermic 

 vesicle before there is any visible sign of the appearance of the 

 secondary area of cell-production. The grey central area is the 

 embryonic disc. The vesicle is expanding approximately equally 

 in all directions ; this is indicated by the concentric circles 

 round the embryonic disc. The dotted line is an imaginary 

 line drawn outside the embryonic area. All parts of the vesicle 

 outside this line are only slightly affected by the subsequent 

 origin of the secondary area of cell-production, as seen by the 

 next diagrams. 



In fig. 41 the secondary centre of activity has become esta- 

 blished, its centre being about the spot marked with a cross in 

 the posterior region of the embryonic disc (grey). The effect 

 is as yet slight, leading to a little more rapid expansion of that 

 part of the embryonic disc indicated by the ellipticity of the 

 lines of growth. This represents the stage of fig. 2, PL 20. 



In fig. 42 the activity of the secondary area of cell produc- 

 tion has become more intense. It is very concentrated, and is 

 now marked by a heaping-up of cells. The outline of this 

 actual thickening must not be taken as being the boundary of 

 the part of the wall of the vesicle due to the secondary centre 

 of activity, for the outermost cells produced thereby will no 

 doubt be stretched and flattened. 



Fig. 43 represents a stage intermediate between figs. 5 and 6 

 on PI. 20. The primitive streak is at its greatest development. 

 The anterior part of the embryonal area, which owes its existence 

 to the primary centre of activity alone, is shown to have its 

 posterior borders distorted as explained in the earlier part of this 

 paper. It is practically the same condition as that illustrated by 

 fig. 42, but more pronounced. The outline of the part of the 

 wall of the vesicle due to the secondary centre of activity is de- 



