218 RICHARD ASSHETON. 



rived from the outline of the primitive streak mesoblast, which 

 upou my hypothesis would approximately mark this area. 



Immediately in front of the anterior end of the secondary 

 area of activity (i.e. in front of the front end of the primitive 

 streak) there will be a lesser tendency to expansion, and 

 accordingly a heaping-up of cells proliferated from the front 

 end of the primitive streak, forming the thickening known as 

 "Kopifortsatz." From this moment the increased intensity of 

 action of the secondary area of cell-production over the primary 

 is shown by the fact that the outline of the former increases its 

 diameter four times by the stage illustrated by fig. 8, whereas 

 the latter's increase is scarcely perceptible. 



Fig. 44 illustrates the conditions of fig. 8. The two centres 

 of growth have become further removed from each other by the 

 interposition of new cell material proliferated chiefly by the 

 secondary area. In other words, growth in length of the 

 embryo has now very markedly taken place. 



This growth in length is represented in the diagram by the 

 area within the curves of which the one marked 1 is the outer- 

 most. The region of the embryonal area due to the primary 

 activity has recovered from its temporary distortion, and has 

 regained its radial symmetry to a great extent. This can be 

 detected in the series of drawings figs. I — 8. 



The secondary area of activity also, upon the diminution of 

 the tension to which it had been subjected, tends also to assume a 

 more radial form (fig. 45, with which fig. 10 may be compared). 

 Ultimately the counteracting effects of the two centres cause 

 each to become tilted over, so that instead of lying in the same 

 plane they lie in different planes parallel to each other, and at 

 right angles to the original common plane. In fact they 

 assume their natural positions. 



On this explanation it is clear that most, if not all, of the 

 ectoplacental region is really derived from the primitive streak, 

 which may perhaps account for its much greater activity than 

 that part of the blastodermic vesicle which arises directly from 

 the primary centre of activity. 



