describes the appearance of a cavity between the outer layer of epiblast and the inner layer of epiblast. It is hardly a cavity, for it is partially filled by very "stellate" cells. With the formation of this the inner layer of epiblast becomes arched inwards, a process termed "temporary inversion of layer" by Heape. Subsequently this arch flattens out again, and it and the outer layer cells and stellate cells between them all fuse together to form the permanent epiblast.

I have not studied the mole, but from Heape's description this seems to be an almost exact parallel to the process which occurs in the rabbit, with this exception: whereas in the rabbit the increase in activity of the inner layer of epiblast gives rise to a rupture of the outer layer of epiblast, in the mole one of the alternatives suggested above is taken, and the inner epiblast bulges inwards, leaving a loose space threaded across with "stellate" cells between it and the zona radiata. Subsequently on the expansion of the whole blastodermic vesicle the plate flattens out again and the "stellate" cells and other outer layer cells become intermingled with those of the plate and form the permanent epiblast. Heape regards this as a kind of inversion, and the stellate cells as "träger."

I cannot agree with Heape in considering the stellate cells he mentions as being equivalent to träger cells, and certainly I do not think that the "Rauber cells " of the rabbit are in any way connected with "träger "—but of this I shall say more in a later paper.

As regards the meaning of the fusion of the two layers, I do not see that it need necessarily have any morphological significance at all. It may be merely an accident of development. At the same time I cannot entirely neglect certain occurrences in another group of Vertebrates, and have discussed them in another paper.

By the union of the two layers the embryonic disc acquires a very much more distinct outline, which is now practically circular; its outline is considerably more regular than before the junction just described has taken place.

This

Hypoblast.-(1) Hypoblast of embryonic area. seems to have become in very slight measure changed. It is now undoubtedly a continuous membrane in the region of the embryonic area. This condition seems to extend a distance from the embryonic area equal to about the diameter of the embryonic area, beyond which it becomes a network and passes insensibly into

(2) The straggling cell portion of hypoblast. This part of the hypoblastic layer retains its irregularly scattered condition of the sixth day, but certain features may be remarked upon which were unnoticed before.

The cells are more thickly scattered about; they are more irregular, having entirely lost their rounded form, and are more flattened. Many in all parts may be seen to be connected together by fine filamentous strands, not only in the close proximity of the embryonic hypoblast, but also near its periphery.

Again, the outer limit of this zone is much more marked, and is, in fact, now rendered very plain indeed. It forms a wellmarked edge, very irregular it is true, but an almost if not quite continuous edge. Along this edge the cells are slightly crowded, and rather elongated in the equatorial plane of the vesicle. What I mean may be made out from fig. 40, and an idea of the general history of events connected with the development of this part of the hypoblastic layer may be derived from the four figures 37-40.

The extent of area covered by the two parts of the hypoblast is now rather more than half the whole area of the inside of the wall of the blastodermic vesicle.

Another point of interest may be noticed. There is a strong tendency for this line of limit to be thrown into small folds or waves, as shown in fig. 40, Pl. 17.

These two characters may be observed thoughout the area under discussion. If an embryo of this age is cut in two, and the cut edge examined under a high power, these characters are seen very clearly. That is to say, the cells forming this limiting line are themselves rather more rounded or "hog

backed," and the joining strands are curved and even arch away from the epiblast, and in some cases undoubtedly the cells themselves seem to stand away from the surface.

It is, however, quite possible that the sinuosity of the line and arching away may be the result of reagents, as I have not examined this edge in a fresh specimen with success,

EXPLANATION OF PLATES 13-17,

Illustrating Mr. Richard Assheton's paper, "A Re-investigation into the Early Stages of the Development of the Rabbit."

LIST OF REFERENCE LETTERS.

4. Anterior end of blastodermic vesicle. ALB. Albuminous layer acquired in Fallopian tube. C. BL. Cavity of blastodermic vesicle. EM. D. Embryonic disc. EP. I. Inner layer of epiblast. EP. O. Outer layer of epiblast. EP. OR. Cell of outer layer of epiblast becoming "accidentally" included in the inner layer of epiblast. HY. Hypoblast of embryonic disc. HY. I. Hypoblast of region beyond embryonic disc. I. M. Inner mass of cells of blastodermic vesicle. L. Larger of the first two segments. L. Supposed second generation of layer of first two segments. 13. Supposed third generation of layer of first two segments. O. L. Outer layer of cells of blastodermic vesicle. P. Posterior end of blastodermic vesicle. P. B. Polar body accidentally enclosed. PL. S. Sinuous protoplasmic junction between two hypoblast cells. S. Smaller of the first two segments. S. Supposed second generation of smaller of first two segments. S3. Supposed third generation of smaller of first two segments. x., x1. Deceptive appearances suggesting a van Beneden blastopore. Z. Zona radiata.

All the figures excepting Figs. 37, 38, and 41 have been drawn with the help of a camera.

All those figures of which the magnification is 465 times were drawn with Powell and Lealand's apochromatic oil immersion; the others with Zeiss or Reichert's lenses.

VOL. 37, PART 2.-New Ser.

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PLATE 13.

FIG. 1.-Fertilised ovum. Rabbit 24 hours.

× 165. FIG. 2. Ovum in two segments, from same rabbit as above. 24 hours. X 165.

FIG. 3.-Ovum in two segments, showing great difference in size, from same rabbit as above. 24 hours. × 165.

FIG. 4.-Ovum in two segments, from rabbit 24 hours. Polar bodies separated. × 165.

FIG. 5.-Ovum in two segments, from rabbit 25 hours; drawn after mounting in Canada balsam. x 165.

FIG. 6.-Embryo in four segments, from same as preceding. 25 hours. × 165.

FIG. 7.-Embryo in five segments. 27 hours.
FIG. 8.-Embryo in eight segments.

x 165.

× 165.

FIG. 10.-Embryo in seven segments. × 165.

FIG. 11.-Embryo in eight segments, showing internally-placed polar body. 39 hours. × 165.

FIG. 12.-Embryo of 47th hour, showing contrast in size of the several segments (seventeen segments). × 165.

FIG. 13.-Isolated segments of same specimen (47th hour). × 165.
FIG. 14.-Embryo (27 hours) in seven segments.

PLATE 14.

FIG. 15.-Embryo (66 hours), showing great difference in size of segments.

FIG. 16.-Section of a specimen preserved in silver nitrate per cent. stained picro-carmine, and cut in paraffin. 72 hours. × 465.

FIG. 17.-Embryo, showing contrast in size of segments.

FIG. 18.-Section of embryo (72 hours) preserved in Perenyi; stain, borax carmine.

;

FIG. 19.-Another section of the same embryo. Both showing what might be a van Beneden blastopore, but on opposite sides!

FIG. 20.-Section through rabbit embryo of the 47th hour. Cut in paraffin. Preserved per cent. silver nitrate 2 minims, water and sunlight hour; no other stain. × 465.

PLATE 15.

FIG. 21.-Section through rabbit embryo of the 77th hour. Removed from Fallopian tube. Preserved in Perenyi; stained borax carmine; cut in paraffin. This seems to be an unusually large specimen. × 465.

FIG. 22.-Section through rabbit embryo of the 80th hour. Removed from uterus. Preserved in Perenyi, and stained in borax carmine; cut in paraffin. x 465.

FIG. 23.-Section through rabbit embryo of the 83rd hour. Removed from uterus. Preserved in Perenyi, and stained in borax carmine; cut in paraffin. × 465.

FIG. 24.-Section through rabbit embryo of the 80th hour. Removed from the same uterus as Fig. 22. Preserved in Perenyi, and stained picro-carmine; cut in paraffin. x 465.

FIG. 25.-Section of embryo of rabbit of the 80th hour. Taken from uterus (same as Figs. 22 and 24). Preserved in Perenyi, and stained in borax carmine, and cut in paraffin. x 465.

PLATE 16.

FIG. 26.-Section of the embryonic disc of rabbit of 96th hour. Taken from the uterus and preserved in Perenyi, and stained in aniline blue black; cut in paraffin. x 465.

FIG. 27.-Section of embryonic disc of rabbit of 96th hour. Taken from the uterus. Preserved in silver nitrate per cent. 25 minims; water and sunlight 2 hours; alcohol; borax carmine; cut in paraffin. × 465.

FIG. 28.-Section of the embryonic disc of a rabbit of the 100th hour. Taken from the uterus, and preserved in Perenyi. Stained in borax carmine, and cut in paraffin. × 465.

FIG. 29.-Section through the embryonic disc of a rabbit embryo of the 103rd hour. Preserved in Perenyi; stained borax carmine. X 465.

FIG. 30.-Section through a portion of the embryonic disc of a rabbit embryo of about the 140th hour. Preserved in Perenyi; stained picrocarmine. The hypoblast is now nearly a perfect membrane in the embryonic area. X 465.

FIG. 31.-A portion of the lower pole of the same section. X 465.

FIGS. 32, 33, and 34.-Portions of sections of the embryonic disc of a rabbit of the 153rd hour. Preserved Flemming strong formula; stained in borax carmine. The fusion of layers is taking place. × 465.