In the Vespertilionidæ Van Beneden and Julin have shewn that the ovum undergoes at the close of segmentation changes of a more or less similar nature to those in the rabbit; the blastopore would however appear to be wider, and to persist even after the cavity of the blastodermic vesicle has commenced to be developed.
Although by this stage, which occurs in the rabbit between seventy and ninety hours after impregnation, the blastodermic vesicle has by no means attained its greatest dimensions, it has nevertheless grown from about 0.09 mm.—the size of the ovum at the close of segmentation—to about 0.28. It is enclosed by a membrane formed from the zona radiata and the mucous layer around it. The blastodermic vesicle continues to enlarge rapidly, and during the process the hypoblastic mass undergoes important changes. It spreads out on the inner side of the epiblast and at the same time loses its lens-like form and becomes flattened. The central part of it remains however thicker, and is constituted of two rows of cells, while the peripheral part, the outer boundary of which is irregular, is formed of an imperfect layer of amœboid cells which continually spread further and further within the epiblast. The central thickening of the hypoblast forms an opaque circular spot on the blastoderm, which constitutes the commencement of the embryonic area.
The history of the stages immediately following, from about the commencement of the fifth day to the seventh day, when a primitive streak makes its appearance, is imperfectly understood, and has been interpreted very differently by Van Beneden (No. [171]) on the one hand and by Kölliker ([184]), Rauber ([187]) and Lieberkühn ([186]) on the other. I have myself in conjunction with my pupil, Mr Heape, also conducted some investigations on these stages, which have unfortunately not as yet led me to a completely satisfactory reconciliation of the opposing views.
Van Beneden states that about five days after impregnation the hypoblast cells in the embryonic area become divided into two distinct strata, an upper stratum of small cells adjoining the epiblast and a lower stratum of flattened cells which form the true hypoblast. At the edge of the embryonic area the hypoblast is continuous with a peripheral ring of the amœboid cells of the earlier stage, which now form, except at the edge of the ring, a continuous layer of flattened cells in contact with the epiblast. During the sixth day the flattened epiblast cells are believed by Van Beneden to become columnar. The embryonic area gradually extends itself, and as it does so becomes oval. A central lighter portion next becomes apparent, which gradually spreads, till eventually the darker part of the embryonic area forms a crescent at the posterior part of the now somewhat pyriform embryonic area. The lighter part is formed of columnar epiblast and hypoblast only, while in the darker area a layer of the mesoblast, derived from the intermediate layer of the fifth day, is also found. In this darker area the primitive streak originates early on the seventh day.
Kölliker, following the lines originally laid down by Rauber, has arrived at very different results. He starts from the three-layered condition described by Van Beneden for the fifth day, but does not give any investigations of his own as to the origin of the middle layer. He holds the outer layer to be a provisional layer of protective cells, forming part of the wall of the original vesicle, the middle layer he regards as the true epiblast and the inner layer as the hypoblast.
During the sixth day he finds that the cells of the outer layer gradually cease to form a continuous layer and finally disappear; while the cells of the middle layer become columnar, and form the columnar epiblast present in the embryonic area at the end of the sixth day. The mesoblast first takes its origin in the region and on the formation of the primitive streak.
The investigations of Heape and myself do not extend to the first formation of the intermediate layer found on the fifth day. We find on the sixth day in germinal vesicles of about 2.2-2.5 millimetres in diameter with embryonic areas of about .8 mm. that the embryonic area ([fig. 136]) is throughout composed of:
(1) A layer of flattened hypoblast cells;
(2) A somewhat irregular layer of more columnar elements, in some places only a single row deep and in other places two or more rows deep.
(3) Flat elements on the surface, which do not, however, form a continuous layer, and are intimately attached to the columnar cells below.
Our results as to the structure of the blastoderm at this stage closely correspond therefore with those of Kölliker, but on one important point we have arrived at a different conclusion. Kölliker states that he has never found the flattened elements in the act of becoming columnar. We believe that we have in many instances been able to trace them in the act of undergoing this change, and have attempted to shew this in our figure.
Our next oldest embryonic areas were somewhat pyriform measuring about 1.19 mm. in length and .85 in breadth. Of these we have several, some from a rabbit in which we also met with younger still nearly circular areas. All of them had a distinctly marked posterior opacity forming a commencing primitive streak, though decidedly less advanced than in the blastoderm represented in [fig. 140]. In the younger specimens the epiblast in front of the primitive streak was formed of a single row of columnar cells ([fig. 138] A), no mesoblast was present and the hypoblast formed a layer of flattened cells. In the region immediately in front of the primitive streak, an irregular layer of mesoblast cells was interposed between the epiblast and hypoblast. In the anterior part of the primitive streak itself ([fig. 138] B) there was a layer of mesoblast with a considerable lateral extension, while in the median line there was a distinct mesoblastic proliferation of epiblast cells. In the posterior sections the lateral extension of the mesoblast was less, but the mesoblast cells formed a thicker cord in the axial line.
Owing to the unsatisfactory character of our data the following attempt to fill in the history of the fifth and sixth days must be regarded as tentative[78]. At the commencement of the fifth day the central thickening, of what has been called above the primitive hypoblast, becomes divided into two layers: the lower of these is continuous with the peripheral hypoblast and is formed of flattened cells, while the upper one is formed of small rounded elements. The superficial epiblast again is formed of flattened cells.