200 RICHARD ASSHETON. 



is a ridge aloug the median line of the area; it still 

 further shortens and diminishes in size, but does not 

 finally disappear until the last segment has been 

 formed (or until sufficient material has been added to allow 

 of the formation of the requisite number of segments). 



The above description draws attention to the most note- 

 worthy features of the primitive streak itself, from the moment 

 of its first appearance. At no time during its existence, or before 

 its existence, is there any trace of any form of concrescence 

 such as Duval has described for the Avian primitive streak. Is 

 it possible to account for the changes as seen in the rabbit by 

 a consideration of the conditions under which the blastoderm 

 of the rabbit is placed ? 



The following pages contain what seems to me a likely ex- 

 planation. 



The Process of Elongation of the Primitive Streak. 



In the exercise of its functions the secondary area of activity 

 is essentially centrifugal in its results. While it is, so to speak, 

 spread out flat upon a plane (as during the stages represented 

 in figs. 3 — 10), the most noticeable feature of its action is the 

 production in every direction of a sheet of mesoblast, of almost 

 circular outline. When after ten days or so it becomes a freely- 

 growing knob, it produces a mass of cells, the outline of which 

 still is circular, the areaitself being now also circular in outline. 



If the nature of the area, as shown by its action and by the 

 form assumed when perfectly free, is to be radially symmetrical, 

 why does it assume a linear form ? Why does it become dis- 

 torted ? Is it an ontogenetic distortion, or is it connected 

 directly with phylogenetic causes ? 



I believe the actual elongation, or rather all the changes it 

 undergoes, including the grooving, is due entirely to the 

 ontogenetic influences, and has no recapitulatory meaning. In 

 one sense it may be said to be due to the phylogeny ; but only 

 in that, unless the mammalia had been descended from animals 

 which had large yolked eggs, the ontogenetic development of a 



