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their nuclei. The first of them to be formed are budded forth at the surface near the lower pole at a time when the "primary mesoblasts" have budded three or four times (Fig. 3, D). Those produced later do not reach the surface, the macromere-nuclei receding from the surface and leaving below them (towards the surface) a closely packed mass or plug of small cells (Fig. 3, E), the more anterior of which have been derived from the macromeres, and, therefore, are unquestionably of entoblastic origin, while the more posterior have been derived from the "primary mesoblasts." This plug is bor

dered in front and at the sides by the ectoblast-cells of the lips of the blastopore, which has now become much diminished in size, while posteriorly it abuts superficially against the ectoblast-cells of the somatic plate (derivatives of “d2” or “X,” the first somatoblast) and at a deeper level against the primary mesoblasts (Fig. 3, E). In the cells of this plug are now developed coarse granules of black pigment (Fig. 3, F), by means of which they are so unmistakably marked that their later history may be followed step by step with great accuracy. Thus arises the pigment-area at the lower pole of the trochophore larva, described in my first paper on Nereis.2

In that paper I concluded that the pigment-cells were derived solely from the "primary mesoblasts," having overlooked the fact described above that a part of them, and probably the greater part, are derived from the macromeres (entomeres). I reached the further conclusion that the pigment-cells wandered into the interior and spread out upon the wall of the archenteron to form a part of the splanchnic mesoblast. Renewed studies demonstrate the erroneous nature of this latter conclusion, and prove that the pigment-cells give rise to the posterior part of the archenteric wall itself. Both in total preparations and in serial longitudinal sections of the successive stages, every step can

1 These cells are obviously comparable to the entoblast-cells of the fourth and fifth quartets (and later entoblast-derivatives) in other annelids. In Nereis they show no definite arrangement.

21892, pp. 412, 417.

3 Nereis, p. 413.

The best results were obtained with strong Flemming's fluid.

be followed of the progressive inwandering of the pigment-cells (Fig. 4) to form the narrower posterior part of the pear-shaped archenteron, while the anterior part is developed from the four macromeres (entomeres) as is proved by the fact, among others, that the fat-drops are found lying in its wall. There is no possibility of mistaking the fact that the pigment-cells actually form the archenteric wall, for their outlines can easily be seen

[blocks in formation]

FIG. 4, NEREIS. Sagittal sections of larvæ. A, trochophore (60 hours), showing inwandering of the pigment-cells at the lower pole; stomodæum and neural plate at the right; B, larva of 41⁄2 days, showing the pigment-cells at p.

and the pigment-granules are found throughout the whole thickness of the wall (Fig. 4, B). The pigment-cells are, therefore, not mesoblastic, but are entoblast-cells.

In so far as the pigment-cells are derived from the macromeres (entomeres), this is exactly what we should expect. That cells derived from the "primary mesoblasts" should enter into the formation of the archenteron is however a surprising result; and it is, therefore, highly important to make

certain, first whether the pigment-cells are in part identical with or descended from the small cells budded forth from the "primary mesoblasts," and second, whether, if this be the fact, the cells of such origin also wander in to form a part of the entoblast. A careful study of the successive stages in surface views, optical sections, and actual serial sections hardly leaves room for doubt in regard to either point. In the first place, pigment is developed in the smali cells that abut directly against the primary mesoblasts (Fig. 3, F), and the products of the latter form so considerable a group that it would hardly be possible to overlook their displacement or wandering away did such a process occur before the appearance of the pigment. I can find no evidence of such displacement and hence cannot escape the conclusion that the pigment-cells lying just anterior to the primary mesoblasts have been derived from them. The evidence on the second point, while perhaps not demonstrative, is hardly less convincing. The pigment-cells disappear from the surface pari passu with the growth of the archenteron; and when the latter is fully formed (in embryos of five days and upwards) not a trace of pigment can be found at the surface or in any of the cells of the posterior region save those of the archenteron. That the superficial pigment-cells actually pass inwards is proved by the fact that from its first appearance the pigment is densest in two (sometimes three) symmetrical areas which are first seen at the surface and may then be traced progressively inwards in the archenteric wall.1

Taken together, these facts leave no doubt, in my opinion, that the pigment-cells are derived in part from the primary mesoblasts, in part from the entomeres, and that the cells from both sources give rise to a portion of the archenteric wall and to no other structure. If this conclusion be correct, it follows that the "primary mesoblasts" are not properly so-called, but are mesentoblasts, precisely as Conklin has described in Crepidula. Now, there can be no doubt that the single pair of minute cells in Aricia and Spio represent the group of cells of like origin in

1 Cf. 1892, Figs. 79-91, which show this fact, through not as clearly as it appears in my more recent preparations.

Nereis. They must, therefore, be regarded as vestiges of functional entoblast-cells such as those of Nercis, and morphologically they represent the posterior part of the entoblast-plate (Cf. Fig. 1, B; Fig. 2, A).

The foregoing interpretation is entirely in harmony with Conklin's important discoveries in the gasteropod Crepidula. Conklin here definitely showed, for the first time in any animal,2 that the so-called "primary mesoblasts "give rise to a group of entoblast-cells before dividing to form the mesoblast-bands. But more than this, Crepidula represents a step in the series which may be regarded as anterior to the condition found in Nereis; for here each mesentoblast divides off two entoblastcells, the bulk of which taken together is actually greater than that of the mesoblastic material remaining, "less than half the cell (4d) being destined to form mesoblast."3 The three forms Crepidula, Nereis, Aricia, thus form a progressive series in which the entoblastic part of the mesentoblast cell is reduced from more than half the bulk of the cell to an insignificant vestige. It is probable that two intermediate steps besides Nercis have been observed by Lillie and Mead respectively. The two cells. found by the first named observer, in Unio, are somewhat larger than those of Nereis; while in Clymenella as described by Mead, they are equal in size to the mesoblastic moiety."

1It would be interesting to determine whether the vestigial cells of Aricia may not be taken into the archenteric wall and thus still retain their functional significance. I have not thus far been able to determine this point; but Mead's observations on Amphitrite seem to show that in this form such is not the case, for the vestigial cells are here formed so far from the surface that they pass into the cleavagecavity and are carried forwards at the tips of the mesoblast-bands. Mead himself concludes that their position in Amphitrite is secondary, being a "reminiscence of a surface division which still persists in many forms" (1897, p. 295) I would sug gest that their position in Amphitrite may be due to the early inwandering of the "primary mesoblasts." It is not surprising that a vestigial cell of this kind should vary somewhat in position; and it should be recalled that in Vereis the later-formed cells lie at some distance below the surface. In Aricia, too, the vestigial cells do not always reach the surface.

2 Compare, however, the somewhat similar earlier accounts of Patten for Patella (1896) and Stauffacher for Cyclas (1893). See Conklin, p. 71.

3

Crepidula, p. 69.

Unio, Fig. 60.

51897, Fig. 88.

Neither of these observers, it is true, suggests the interpretation given above, Lillie somewhat doubtfully assigning to the superficial cells the same fate as I originally did in Nereis, while Mead leaves the matter undetermined. It seems probable, however, that we may look for the same fate for these cells as in Crepidula or Nereis,' indeed I venture to think that Lillie's observations are themselves open to such an interpretation.

These facts, I believe, support the view which has been held by many embryologists from the time of Kowalevsky onwards 3 that the primary mesoblasts, or mesoblastic pole-cells of annelids and mollusks must be regarded as derivatives of the archenteron. In both these groups the primary mesoblasts are derived from the posterior cell of the fourth quartet of "micromeres," the lateral and anterior cells of which are, so far as we know, strictly and always entoblastic. The facts indicate, further, that a progressive process of differentiation in cleavage has been going forward, through which the posterior cell of this quartet has become more and more strictly given over to the formation of mesoblast. The vestigial cells of Aricia, Spio, Amphitrite and Planorbis would seem to represent the last traces. of such archenteric origin of the teloblasts; and it is possible, indeed probable, that there are cases in which even these traces have disappeared, the posterior cell of the fourth quartet being strictly mesoblastic from the first.*

1 Conklin has fully considered (Crepidula, p. 72) the apparently contradictory case of Umbrella, as described by Heymons (1893), where cells exactly corresponding to the "posterior enteroblasts" of Crepidula are described as giving rise to mesoblast. Despite Heymon's careful account, I venture to think that the case demands re-investigation in the light of Conklin's work. In a recent account of the mesoblast in Physa (1897), Wierzejski finds that small cells ("mesoderm-micromeres') are budded forth not only from the "primary mesoblasts" but also from the larger lateral cells derived from them. All these cells are assumed to be mesoblastic, though their fate was not followed out (1897, p. 391).

2 Unio, Fig. 67.

3 Cf. Kowalevsky, 1871, p. 30; O. and R. Hertwig, 1881, p. 47. Hatschek, 1888, p. 76; Rabl, 1889, p. 207, and earlier literature there cited.

This point must remain doubtful until renewed investigation shall show whether the superficial budding is ever entirely suppressed; for we cannot safely infer its absence from existing accounts, and I am not convinced that my own statement of their apparent absence in Polymnia (Nereis, p. 458) may not have rested upon an oversight.

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