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formation of the primitive archenteron is by a process of splitting, and is the direct effect of the primary centre of growth; whilst the continuation of the cavity produced by an overgrowth is the direct effect of the secondary centre of growth, producing the elongation of the animal.
The splitting process in the frog corresponds in results to the invagination process of Amphioxus, while the overgrowth of certain parts of the white pole of the ovum of the frog by the dorsal, and subsequently lateral and ventral lips of the blastopore, together with the continuation of this process in the formation of the tail, corresponds to the elongation of the gastrula in Amphioxus, by means of what Hatschek called the polar cells.
I shall now attempt to explain what I believe to be the actual method in which the splitting is brought about.
The frog's egg segments, as has been described by many observers, more rapidly at one pole than the other. This is, I think, universally supposed to be due to the greater accumulation of yolk granules at the " lower” pole, which thereby hinder the segmentation activity at that pole.
If we admit that "yolk ” determines the inequality of the process known as segmentation, we must admit it also in the case of each cell. If it is true of the segmented ovum, it is equally true of the unsegmented ovum. To say that yolk being more plentiful in one part of a cell than in another hinders the activity of the protoplasm, is the same as saying that a cell divides into two parts, which in magnitude are in inverse ratio to the purity of the protoplasm contained. In other words, the result of a simple process of cell division, such as we see in the segmenting ovum, is two cells equally balanced as regards protoplasmic energy.
Fig. 15 on Pl. 24 is a diagram of a vertical section of the unsegmented ovum of the frog.
The circles 1 to 7 represent diagrammatically what I imagine to be the distribution of yolk, as determined from a consideration of the segmented ovum.
The space No. 1 is that region in which segmentation is most retarded, and so presumably the region in which yolk is
most abundant. The space No. 2 contains less yolk to a given area than No. 1, No. 3 less than No. 2, and so on.
For the sake of simplicity we may regard the outer space only. This may be supposed to contain protoplasm of a uniform degree of purity. Accordingly division of this space will be such as to produce two spaces whose areas are equal. This is about the spot marked by the line (a), and will represent the third furrow of segmentation, that is the first horizontal furrow.
Similarly, the next horizontal furrows will be about the spots 6 b, the next at cccc, the next at d d d d d d d d, and so on; always resulting in a balance of protoplasmic energy on each side of the furrow.
In this way the frog's egg becomes segmented more and more rapidly in the upper hemisphere than in the lower. For a considerable time there is an almost complete absence of horizontal furrows in the lower hemispheres.
This point is very well seen in Umé Tsuda's figures iv, v, of Plate 24, 'Quart. Journ. Micr. Sci.,' vol. xxxv, part 3.
Another effect is that as segmentation proceeds there is a continual increasing disparity in size between the cells of the black pole and those of the white. Whereas at first the superficial area of the cells of the extreme upper pole bears to the superficial area of the cells of the extreme lower pole the ratio of 1 to 2, at the time of the commencement of the blastopore it bears the ratio of 1 to 5.
In this way there is a gradual apparent creeping of small (black) cells over the surface of the egg-though in reality it is conversion of large cells into smaller in situ, as, I believe, is now generally accepted.
My diagram fig. 15 gives the idea of no segments in the white or lower hemisphere of the ovum. This is because it deals only with horizontal furrows.
The segmentation energy may be said to produce its effects along the area of least resistance. Is it not possible that the commencement of the archenteron may be a continuation of this same process ?
The effect up to now has been to produce a fairly sharp line of demarcation between small and large cells upon the surface at the point x in diagram, fig. 15.
On the supposition that this diagram represents fairly accurately the distribution of yolk, it is clear that as this line advances it encounters greater and greater resistance. May not a time come when it will find the path of least resistance to be inwards and backwards, as in diagram 16?
Diagrams 15 and 16 are inaccurate for later stages of segmentation, because they do not show a segmentation cavity.
Fig. 12 is a more accurate representation of a completely segmented egg.
A. is the black upper pole (the anterior wall of the future embryo); P. is the white lower pole (posterior end of the future embryo); sg. the segmentation cavity.
The letter A. points to the smallest cells of this stage, y. p. to the largest.
There is a gradual merging of the one into the other, not along the surface, for here the line is much sharper, but along the cells to which y. and x. are directed.
My idea is that the continuation of the segmentation process is the conversion of, first, the cells y., then the cells x. into smaller ones, and in this way a layer of small cells will be produced lying up against the mass of much larger cells y. p.
This layer I have indicated in the fig. 12 by the dotted line.
If a section of an embryo of the stage in which the blastopore is nearly complete is examined, it will be seen that there is such a layer of cells along the floor of the segmentation cavity.
Fig. 21 is an outline camera drawing. Such details as are shown were not drawn by camera. The smallest cells are those forming the lip of the blastopore.
The point a, represents the at present most anterior limit of the archenteron. More anteriorly, however, following the lines a a., a a., there is what I take to be a differentiation of the yolk-cells, that is a splitting up into smaller cells, which cells, upon the splitting hypothesis, will eventually form the
roof of the archenteron, and come to lie up against the epiblast now forming the roof of the segmentation cavity, as shown diagrammatically in figs. 12 and 13.
The differentiation as seen at the point a a. must, on this hypothesis, be considered to be the effect of the direct continuation of the process of differentiation on the surface of the ovum (whereby the epiblast is separated), that is a direct continuation of the process of segmentation. This line bounded above by small cells, below by large cells, constitutes a line of separation or a split, which is, I believe, the first commencement of the archenteron, and is a result of the primary centre of activity, comparable to the events of the first five days in the development of the rabbit, or to the formation of the gastrula in Amphioxus. But although corresponding in effects, the only really homologous feature is the presence of a primary centre of activity, or process of segmentation of the egg; the actual directive agencies being in each case cænogenetic and entirely different.
The conversion of the narrow slit into a spacious cavity is to be considered to be due, at any rate in part, to the effect of the secondary centre of activity, to which I shall refer again.
I must now refer to the experiments made by Roux and Schultze and Morgan and Umé Tsuda upon the developing egg by following natural or artificial spots, which experiments I have myself repeated during this spring.
It is impossible to repeat these experiments without becoming convinced that there is a change of relative position between certain spots on the ovum,- for instance, the dorsal lip of the blastopore, and the most inferior spot upon the white pole of the ovum. These two spots, as seen from without, undoubtedly approach one another before the complete formation of the circular blastopore. But the question to what extent this approximation is carried, and whether by a concrescence of the lateral lips of the blastopore, or by a rolling under of the white pole, or by a growth over the upper lip without concrescence, is answered differently by the several observers.
My own suggestions are as follows,
The Secondary Area of Cell Production. I think that every one will agree that after the closure of the blastopore, the embryo grows in length by the proliferation of cells at the spot which formerly formed part of the lips of the blastopore.
There is very little doubt that rapid growth at this spot takes place before the final closure of the blastopore; the question is, when does this growth begin?
Again, it must be remembered that growth of the embryo as a whole, derived from the rapid multiplication of the cells in this area, is growth in length. It is the secondary area of growth comparable to the secondary area of growth or primitive streak of the rabbit.
If there is such a growth backwards of the blastoporic lips before their closure, there will then be a portion of the future gut cavity of the embryo that will have been formed, not by a splitting nor by an invagination, but by a growth backwards of tbe blastoporic lips.
Amphioxus, after the completion of the process of invagination, begins to grow in length. According to Hatschek's account this was largely due to the activity of two pole cells. Recently, however, Wilson has stated very clearly that these pole cells are "a myth.” They never exist at any time, but the posterior region of the larva of later stages “is rapidly growing, and numerous mitoses may be observed in all the cells in the region of the mesenteric canal."
Now although the process of invagination produces the double-layered condition of the embryo of Amphioxus, and at the same time the cavity of the archenteron, yet it is only the anterior part of the archenteron that is formed in this way.
There is a posterior point of the archenteron which is formed, not by invagination, but by growth of the blastoporic lips. This must be so, whether we accept Hatschek's or Wilson's description of the secondary growing point.
The exact line of demarcation between the two parts I have no means of showing. It is not easy to say at what moment