THE PEIMITIVE STREAK OF THE RABBIT. 191 



The Primitive Streak of the Rabbit ; the Causes 

 which may determine its Shape, and the 

 Part of the Embryo formed by its Activity. 



Ricliard Assheton, M.A. 



With Plates 20—22. 



The following pages are offered as a contribution towards 

 the elucidation of the mode of formation and the function of 

 the primitive streak in the rabbit. 



Although it is necessary to make incidental remarks upon 

 the formation of the mesoblast and notochord, I have not given 

 here a full description of the results of my own observations 

 upon those points, but I hope to be able to do so^ and to dis- 

 cuss the results of former authors upon this and other animals, 

 in a later communication. In the present paper I have confined 

 my remarks almost entirely to the rabbit, and to my own sugges- 

 tions towards an explanation of certain phenomena connected 

 with the structure we call the primitive streak in that animal. 



Of recent years the theory of concrescence has been again 

 brought into great prominence in attempts to account for 

 the growth in length of the Vertebrate embryo. The 

 present paper will, I hope, tend to show that there is no 

 trace of such an occurrence in the rabbit, and that the growth 

 in length of the embryo can quite as well — and to my mind 

 infinitely more easily — be accounted for by a process of addi- 

 tion of new cellular units between the pre-existing embryo and 

 an area of rapid cell-production. I have attempted to show 

 which part of the embryo is to be regarded as the " pre-existing 

 embryo,^' and which as being due to the activity of the area of 

 rapid cell-production — the primitive streak. 



