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upon, or just within the posterior border of the previously circular embryonic disc (see PS., Pl. 20, fig. 2).

Whether this activity concerns in the first instance both the cells derived from the outer layer of epiblast and those derived from the inner layer of epiblast equally, I cannot form at present a definite opinion.

Very soon the centre of this ragged area becomes thickened, and the embryonal area has now the appearance shown in fig. 3. The original outline of the embryonic disc is fairly distinct, but of course slightly larger than in the earlier stage (fig. 1). This thickening, which is due to a greater accumulation of cells at this point, would seem to be the result of a greater activity of cell multiplication at this spot.

As to the cause of this sudden increase of activity I have no suggestion to make. It is, of course, quite possible that the activity was present from the first, and that it is only owing to changed conditions that it now becomes evident as a heapingup of cells. To give a strictly epigenetic explanation some such cause ought to be adduced, but for the present I am not able to suggest any, and so must ascribe it to a palingenetic cause.

It is not possible to point out the exact boundaries of this area of increased activity, but probably if that portion of the epiblast which is distinctly thickened is taken to be the area of increased activity, I think the error will not be great. Up to now, and for some considerable time to come, there is no change noticeable in the condition of the hypoblast.

The darkened area PS. in fig. 3 may be taken as the area of increased activity. The posterior border of this area is semicircular, the anterior border is conical. The darkest part of this area is about the position of the posterior border of the original embryonic disc.

Fig. 4 is a slightly later stage. The anterior portion of the embryonal area still shows a circular darker part, which I take to correspond approximately to the original circular embryonic disc of the sixth day. The secondary area of activity is of the same nature as in the preceding figure, but its anterior conical part is slightly more pronounced. On either side of it there are VOL. 37, PART 2.-NEW SER.

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two parts which are very much less opaque, but still more opaque than the extra-embryonic part of the blastodermic vesicle.

The specimen of which fig. 4 is a drawing was cut into a series of transverse sections, that of which fig. 4A is a drawing, into a series of sagittal sections. Fig. 17, Pl. 20, is a very nearly median sagittal section of the specimen represented by fig. 4 A. In this drawing A. is the anterior end, and P. the posterior.

The epiblast (EP.) of the embryonic disc is thickened throughout. The under or inner surface of the epiblast is even along the anterior half, while along the posterior it presents a very irregular, and towards the extreme posterior end, broken edge.

The hypoblast (HY.) is separate all along, and forms a continuous layer. It is specially thickened at the two points where it underlies the extreme anterior and posterior ends respectively. There are now between the hypoblast and the epiblast, towards the posterior end, certain scattered cells, some apparently entirely, others only partially separated from the epiblast. These are the first mesoblast cells which make their appearance. The figs. 13-16 on Pl. 21 are from transverse sections of the specimen shown in fig. 4. They are drawn on a larger scale, and, taken together with section 17, demonstrate pretty accurately the structure of the specimens figs. 4 and 4 A. Fig. 13 is taken through the anterior part of the specimen along the line 13 in fig. 4. The central portion only is given in fig. 13 (see fig. 13, 4.). This section carries out the evidence derived from the longitudinal section (fig. 17), and shows that the under or inner surface of the epiblast is quite smooth anteriorly.

There is no median thickness of the hypoblast, and no sign of mesoblast cells. In fact the anterior region of the embryonal area has not greatly altered from the condition of the circular embryonic disc upon the day previous. The posterior end, however, is very different. Fig. 16 passes through the specimen (fig. 4) along the line 16-that is to say, through the very densest portion of the area.

Here we see the epiblast enormously thickened, and a certain

number of cells seem to be lying separate or forming a loose network between the epiblast and hypoblast. These are mesoblast cells.

As the sections pass more posteriorly this primitive streak knob becomes smaller and ends rather abruptly, as seen in the sagittal section, fig. 17. But forwards the thickening of the epiblast is continued much further, getting less very slowly, as figs. 14 and 15 show, which are taken along the lines 14 and 15 in fig. 4. This narrow band of thickened epiblast is seen as the irregular lower surface of the epiblast in fig. 17. It extends very nearly halfway, along the length of the embryonal area.

I think it is pretty clear that this is part of the same area of proliferation as that through which fig. 16 passes, as will appear more certainly later on. Hence the most anterior point to which this reaches is a point of very great importance. This is marked A. PS. in figs. 3 and 4.

The proliferation of cells is so rapid at the posterior end as to cause an eminence in transverse section (fig. 16; evident also in fig. 17). The three sections, figs. 14, 15, 16, differ only in amount of proliferating area and accumulation of cells proliferated. This I take to mean a difference only in intensity. Excepting in intensity there is no difference suggested by the structure of the secondary area of activity in the three sections.

The secondary area of proliferation is now fully established, and has changed in shape from an ill-defined spot in fig. 2 to a more and more elongated area in figs. 3 and 4.

In fig. 5 it has still further lengthened. Figs. 21, 22, 23 are sections taken across this specimen at corresponding spots to figs. 14, 15, 16 respectively, and must be compared one with the other. It may be noticed that there is here extremely little difference in the extension transversely of the area of proliferation. The cells, which appear to be entirely separated, extend further from the middle line on each side, but the area of fusion itself (between epiblast and mesoblast) differs very little. This is especially noticeable in the most

posterior of the sections, namely, figs. 16 and 23. The actual area of proliferation is about the same.

In the anterior sections, if there is a difference, it is that in the more advanced specimen (fig. 5) the sections show a smaller extent of proliferating area than in the younger specimen.

In specimen fig. 5 there is now a very slight groove along the surface of this area, so slight as to be scarcely recognisable in sections fig. 22 (P. GR.).

There is another point of interest. Up to this moment the extreme anterior end of the secondary area of proliferation has almost shaded away imperceptibly into the epiblast of the future neural plate, at any rate it has always been the most slender part of the structure. Now it has become a very well-marked spot-it forms almost a knob-and is henceforth recognisable as what other authors have called "Hensen's node.” Fig. 21 passes through the centre of this.

Fig. 6 is a later stage still. In this the secondary area of proliferation has attained about its maximum length, and at this moment shows the greatest degree of distinction into different parts: (i) hindermost, a broad area of proliferating surface, its posterior border forming an arc of a circle, while anteriorly it tapers off into (ii) the middle region, a long narrow strip of proliferating area deeply grooved along its length; more deeply grooved anteriorly and less posteriorly (fig. 26). This again passes abruptly into (iii) the most anterior part of the primitive streak or Hensen's node, a very much thickened and compressed region of short extent (vide section, fig. 25).

This stage is the height of development of the primitive streak. The secondary area of proliferation is at this stage a perfectly typical primitive streak, with well-developed primitive groove.

In the preceding figure (fig. 5) the groove is very shallow and wide, and easily recognisable in the whole specimen. In fig. 6 the groove is very deep and narrow, and is only with difficulty seen in surface views as its walls are nearly approximated, and it appears when seen by transmitted light as a

narrow dark line instead of a wide lighter line when seen under similar conditions at the earlier stage.

In all the stages described hitherto there has been no fusion of the hypoblast with the proliferating epiblast. In stage fig. 5 at the extreme anterior end of this area, where there is a special thickening of the primitive streak, the hypoblast is in very close contact (see figs. 20 and 21).

In this stage (fig. 6) the fusion is complete, and anteriorly the hypoblast in the middle line and the epiblast and mesoblast are continuous in the mass spoken of as Hensen's node.

Figs. 24, 25, 26, and 27 are sections taken along the lines 24, 25, 26, and 27 in fig. 6.

A section taken more anterior still than fig. 24 corresponds to figs. 13 and 19.

The last three sections (figs. 25, 26, and 27) should be compared with figs. 21, 22, and 23, and figs. 14, 15, and 16. Here again it will be noticed that there is no great increase or diminution of extent in transverse section of the area of active proliferation. The most posterior parts differ hardly at all except in the spreading of the separated cells (mesoblast).

It must be noticed that there is now a region hitherto not to be found. The section drawn (fig. 24) passes through this region, which is just anterior to the primitive streak.

The section through the middle region (fig. 26; compared with figs. 22 and 15) differs in one particular only (besides extension of mesoblast). Instead of the area of proliferation appearing at the surface on a level with the rest of the outer layer, it lies at the bottom of a deep groove. The sides of the groove are not "fused" with mesoblast; it is only the floor of the groove that is participating in the addition to the mesoblast.

Fig. 25, the section through the anterior region of this area of proliferation, shows an intensified form of the state in the preceding stage (fig. 21). Here the groove suddenly ceases, and even in most instances gives rise to an eminence. This eminence so overhangs the groove posteriorly as to give rise to

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