1. A series of ten blastocysts of Bettongia cuniculus is described, covering a range of development from the appearance of the primitive streak to the establishment of the medullary plate.

2. In the first blastocyst there has appeared a small area of ectodermal thickening between the centre of the embryonal area and its periphery, and it is suggested that this is the primordium of the primitive streak. In the second blastocyst a very young streak is present which does not reach either to the middle or to the periphery of the area, and whose centre is in nearly the same position as that of the first blastocyst. At its maximum the streak stretches about two-thirds of the way across the embryonal area. A groove develops down its middle and active mesoderm production occurs from its edges. The mesoderm forms an approximately oval area round the streak, and extends into an extra-embryonal position first posteriorly and then laterally.

3. At the anterior end of the streak appears a broader, thicker region of similar but looser tissue, the primitive knot; from here grows forward the head-process in the usual way, and at the same time the endoderm underlying these two structures fuses with them. No lumen is present in the head-process at any stage. The originally circular embryonal area elongates until it becomes somewhat pear-shaped, and later it constricts in the middle.

4. The thickened ring of endoderm at the edge of the embryonal area, the annular zone of proliferation of Hubrecht, is distinguishable at the time of appearance of the primitive streak; but the second thickened area, the prochordal plate, does not appear until the streak is well established, and except in its earliest stages is hard to delimit owing to its relative thinness and to the closely investing mesoderm which soon covers it. The question of mesoderm production from these sources is discussed, and it is concluded that while such does in fact occur in this form it is quite insignificant in amount as compared with that produced from the sides of the primitive streak.

5. In the last stage described the medullary plate and chorda are well established, and the importance of the primitive streak is diminishing. Mesoderm, whose production now appears to be occurring chiefly by division of pre-existing mesoderm cells, is very much more abundant, and can be divided into three parts: thick sheets extending from near the axial line to beyond the edge of the medullary plate, condensations in which the pleuropericardial coelom is appearing anteriorly, and vascular mesoderm in which blood-vessels are forming. Blood-islands are not present within the vessels, and apparently are represented by groups of cells between the vessels. A well-formed prochordal plate is also present.

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