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Ric Assheton del

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The Primitive Streak of the Rabbit; the Causes

which may determine its Shape, and the Part of the Embryo formed by its Activity.


Richard 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 discuss 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 suggestions 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 addition 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.

I have also tried to explain how the changes in shape of the primitive streak are brought about.

I begin with a short account of this area of activity (which I shall speak of as the secondary area or centre of cellproduction-as compared with the process known as the segmentation of the ovum, which I call the primary centre of cell-production) which adds certain details of interest to the accounts already given of the rabbit primitive streak by other authors, notably by Hensen, Kölliker, and Rabl.

The Earliest Signs of the Formation of the Secon

dary Area of Proliferation. Until the middle of the seventh day the embryonic disc is circular in outline. After the fusion of the inner and outer layers of epiblast, the outline of the embryonic disc becomes very much sharper. This state continues until about the middle of the seventh day, when, although the anterior periphery of the embryonic disc retains this character, the arc of the hindermost quadrant becomes very much less distinct, and is no longer a semicircle.

On measuring the embryonic disc very soon after this, it is found to have long and short axes, the long one usually parallel to the long axis of the vesicle, the short to the short axis of the vesicle. The lengthening has been in the antero-posterior direction. The anterior end presents much the same shape and features as before, except that its periphery describes an arc of a larger circle than before. It is still sharply defined. The posterior end is, if anything, more ragged, and is undoubtedly at this moment thinner than the rest of the embryonic area. The measurements are as follows: Blastodermic vesicle, long axis

4:1 mm. short axis

polar axis

Embryonic area,
long axis

transverse axis

1.25 This ragged border I believe to be the effect of the setting up of an area of increased activity of cell-production either

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