scattered hypoblast cells have now become much more numerous, and are scattered more evenly over the portion of the wall upon which they are to be found. Many of them, possibly all of them, are now undoubtedly connected by more or less fine protoplasmic threads.

These scattered cells, although such conspicuous objects during the fifth day, are now extremely difficult to make out, and can very easily escape notice.

They are more numerous now nearer to the embryonic disc, and merge gradually into the continuous layer just described. They are now very much more flattened.

Towards the line of their outer limit they present more the characters of the former day, being fewer and rather rounded and more isolated.

Consideration of the Extent to which the Shape of the Cells of the several Layers may be attributed to Mechanical Causes.

At this time we find cells of two very different types, with cells showing all intermediate stages between the two.

The first type is the rounded, almost completely isolated cell, such as those of the inner layer of epiblast, or near the outer limit of the hypoblast; the second type is the flattened or stretched cell of the outer layer of epiblast or embryonic hypoblast, continuous with its neighbours around all its edges; and thirdly, forms of cells intermediate between these two types. How far can we hold this difference of form to be due to the environment of the individual cell apart from its own inherited tendencies?

Whether there is any cell in the embryo at this time quite separated from all others I am not certain. Of course they are all contiguous to one or more other cells, but possibly there is an actual protoplasmic union between one cell and another. This is certainly the case very frequently with the cells of the hypoblast, which at first sight seem to be quite separate.

In sections, and in surface views, there is no distinct connection to be made out between the rounded cells of the inner

epiblast layer. But when the embryonic disc is broken up in such a way as to scatter and tear apart the various cells, then there undoubtedly appear to be strands passing from some of these rounded cells to others of the same layer. At the same time I cannot say positively whether these strands are really connections between the cells in question, or whether they are fragments and shreds derived from the tearing apart of the hypoblast or outer epiblast layer, between which they are placed. But however this may be, the cells of the inner epiblast layer are, at the time I am speaking of, either isolated or else connected only at certain spots of small area. These are of the rounded type.

At the outer limit of the hypoblast there are also cells, some of which, I believe, may be quite isolated; others are connected to each other by a few strands of protoplasm. These approach very closely to the rounded type of cell. This is the type of cell which I believe to be the most natural, by which I mean the least influenced by its environment. This is the type of cell which first comes into existence in the segmentation of the ovum, when, within the protecting investments, the cells, at first uninfluenced by pressure or tension from without, or from each other, assume their natural or spherical contour. As the segmentation proceeds, the inner segments become the more compressed, and assume polygonal forms.

After the establishment of the cavity of the blastodermic vesicle, the outer cells, by the pressure from within increasing more rapidly than their rate of multiplication, are drawn out into thin plate-like cells.

These outer layer cells are from the first connected with each other by their edges, and form a continuous membrane, a condition without which in all probability the formation and enlargement of the blastodermic vesicle could not be produced.

As long as this tension within is maintained at a rate greater than the rate of multiplication of the cells, the cells retain their flattened condition.

What of the inner mass cells? Upon the removal of the pressure of the outer layer, the more outwardly placed of the

inner mass cells re-assume, at any rate on their free surfaces, the rounded contour which is natural to them.

As the blastodermic vesicle expands, the inner mass, which is adherent to the wall of the vesicle either by actual protoplasmic connections or otherwise, is drawn out into a lenticular shape.

I have tried to show that there is a zone of the wall of the vesicle which, by greater activity of multiplication of cells, admits of more rapid expansion of that part. Upon this zone rests the edge of the lenticular inner mass. The expansion of the zone is in direction radially from the embryonic poles. Hence the outermost cells of the inner mass, i. e. the cells at the edge of the lenticular mass, will tend to be separated more rapidly than the innermost. This will tend to isolate these cells from others of the inner mass. Let us suppose that all the cells of the hypoblast layer are dividing at a uniform rate. I think it is reasonable to suppose that the existence of strands connecting cells of this kind together have their origin in past cell divisions. Accordingly on this supposition the connecting strands will be more numerous and the nuclei nearer together, and the meshes of smaller area in the embryonic disc hypoblast than in the hypoblast outside that area. The hypoblast cells of the embryonic area will differ from those of the extra-embryonic area in this way:

(i) The embryonic cells will have more and shorter, and so presumably stronger, strands connecting them with their neighbours than will the extra-embryonic hypoblast cells.

(ii) The embryonic hypoblast cells will have connecting strands upon all sides, whereas the outermost extra-embryonic cells will have them upon one side only.

Is it possible for the flattening of the embryonic hypoblast cells to be due to their becoming stretched by the tension. produced by the extra-embryonic hypoblast cells (with which they are in direct connection by means of the strands just mentioned) being removed in all directions by the rapidly expanding zone of the outer epiblast? If so, it is possible to account for all the shapes of the cells composing the embryo at this age.

As I have stated on a previous page, the hypoblast of the embryonic area is a network at first. Also, I believe that at first many of the outermost cells of the extra-embryonic area are really isolated. These will be under less tension than those near the embryonic pole, as they will, if they are connected at all with other hypoblast cells, have connecting strands upon one edge only. Hence these preserve for a longer period their rounded contour.

The ultimate conversion of the isolated cells into a network (or a series of networks) and of the networks into thin continuous membranes, and of thin continuous membranes into columnar membranes, would seem, therefore, to be but the result of increase of rate of multiplication over rate of expansion.

The inner layer of epiblast cannot be said to have come into existence as a layer until after the formation of the hypoblast. Until that moment it formed, together with the future hypoblast, the inner mass. It was impossible, except in as far as could be premised from their position, to say from their characters which would be inner epiblast cells and which hypoblast cells (v. fig. 28). What I believe takes place is this. Those cells of the lenticular inner mass which, being at its edges, are removed by the expansion of the wall of the vesicle, and those which are in direct connection with the cells so removed, become by virtue of their position the future hypoblast; the remainder become the inner layer of epiblast. That is to say, those cells of the inner mass which are not influenced by the expansion of the vesicle, as above described, and are accordingly upon that part of the wall, though not actually as yet part of it, which is least affected by the expanding forces, become the inner epiblastic layer.

Of all the cells, therefore, in the embryo at this time, these (the inner epiblast layer) are least affected by external causes. These cells are the more free to assume their natural shape, which I believe to be spherical, and are only slightly flattened between the two layers, outer epiblast and hypoblast.

CHAPTER V.

CHANGES THAT OCCUR DURING THE SEVENTH DAY (145TH TO 168TH HOURS).

The Fate of the Outer Layer of Epiblast (or Rauber's Layer) in the Embryonic Disc.

The embryos have now grown to such a size as to cause them to respond more effectually to the impulses set up by contractile movements of the muscular walls of the uterus, and therefore we find them much further advanced along the uterine tube, and more scattered. They have not, however, taken up a permanent position as yet, for although this may occur in some cases during the last few hours of the seventh day, more usually it does not take place until the early part of the eighth day.

It will be best to describe the course of events in the three layers separately.

Outer Layer of Epiblast.-Very little need be said of the greater part of this layer, no change except such as has been described as occurring during the fifth and sixth days takes place. But special attention must be given to that part of the area which lies over the patch of inner layer of epiblast, i.e. embryonic disc.

Inner Layer of Epiblast.-During the early part of the seventh day the cells of this layer are just as described in the preceding chapter. They extend now over an area of about 6 mm. The general outline of the mass is still circular. Each cell is distinct and rounded, with very large nucleus; and with nearly all stains that I have used they stain only lightly.

Early on the seventh day these cells show signs of greatly increased activity. They multiply, become pressed together, and now form a very compact layer at the same time as certain changes occur in the outer layer of epiblast.

The course taken by these two layers during the next few hours, and its significance, have been very differently described