June 1911 NOTE ON THE BLASTODERMIC VESICLE OF CHARLES S. MINOT From the Laboratory of Comparative Anatomy, Harvard Medical School TWO FIGURES The publication of Professor Hill's very valuable monograph1 on the early development of Marsupials renders it advisable to report on the structure of several opossum blastocysts of about 1.0 mm. diameter belonging to the Harvard Embryological Collection. In structure these vesicles agree essentially with the descriptions of Professor Hill, although in a stage, which is intermediate between those studied by him. They afford confirmation and some extension of his important results. Two of the vesicles were presented by Professor McClure, whose generosity it is a pleasure to acknowledge gratefully. They had been very successfully preserved with Flemming's fluid. One of these was torn with dissecting needles into several fragments, which were then mounted in damar, to furnish surface views; it forms no. 1697 in our collection. The other is to be cut into serial sections for the Harvard Embryological Collection. The remaining vesicles were collected by Dr. J. L. Bremer from a single female. They were preserved in Zenker's fluid. Two of them were found in series 835, of a pregnant uterus. Two were removed from the uterus before preservation. All of the specimens are close to 1.0 mm. in diameter, but the size of the two cut in utero is estimated only, for they are somewhat distorted and of course have been made smaller by the action. of paraffine, in which the specimen was imbedded. The vesicles 1 Quarterly Journal of Microscopical Science, vol. 56, p. 1-134. December, 1910. 295 are all enclosed by a thin zona pellucida, which appears homogeneous. In the sections its outer surface is seen to bear widely separated minute protuberances each about the size and somewhat of the form of an endothelial nucleus, but having the same optical appearance as the substance of the zona. Sometimes outside the zona there is a thin badly stained layer of cells, which may be a remnant of the corona radiata. In the alcoholic specimens the zona is smooth and forms an almost perfectly spherical vesicle. In the sections it is more or less crumpled. The blastocysts are completely didermic throughout, but I find no trace of a middle layer, or of anything resembling a primitive (or so-called Hensen's) knot. The cellular layers are fitted close against the zona, although in the sections they appear artificially and irregularly separated from it. The inner layer is complete, or nearly so, is thin and has the nuclei widely spaced. The outer layer is complete, but comprises two areas, one smaller, the other larger. The smaller area forms an almost circular polar cap, which occupies about one-eighth of the surface of the ovum, but varies in size in the different vesicles; it is conspicuous owing to its darker appearance. The larger area completes the blastocystic wall and is lighter in effect, and further differs from the smaller area by having scattered over it light spots, which viewed with a hand lens seem to be minute round holes. It seems safe to conclude that we have to do with a true ectoderm, an entoderm and an embryonic shield in an early stage. The shield is nearly circular in outline. Viewed from the surface, fig. 1, B the nuclei appear closely crowded together, varying considerably in form and diameter, but all rounded in outline. The figure suggests that they are all at one level, but this can be demonstrated by the use of the fine adjustment not to be the case. In my book "Age, Growth, and Death," p. 175, I have pointed out that the nuclei, specifically of the rabbit, pass through definite ontogenetic stages. In what there is given as the first stage, fig. 61, nos. 1, 2, 3 and 4 the nuclei have a network structure, and one, two or three nucleolus-like or plassome masses. In the next stage, nos. 5-8, the network has become coarser and the meshes wider. All the nuclei in all the layers in these opos sum resemble in structure the first stage of the rabbit nuclei. In the surface view of the shield of the opossum, I am unable to recognize any cell boundaries, which can be identified with certainty as such, but the reticulated protoplasm is easily observed between the nuclei. With an oil immersion objective and careful focusing, one can observe the entodermal nuclei of the shield; they can be easily recognized by their greater diameter. They are quite widely separated from one another. The edge of the embryonic shield is very sharp. I have noticed no alteration Fig. 1 Vesicles of 1.0 mm. diameter of opossum. A. Section of the embryonic shield. z, zona pellucida. ec, ectoderm. ent, entoderm. H. E. C. Series 835, section 30. B. Surface view of embryonic shield. H. E. C. Series 1697. C. Entodermal nuclei of extra-embryonic region. H. E. C. Series 1697. All X 500 diams. in the entoderm, the transition being effected apparently only by alteration of the ectodermal cells. Around the edge of the shield is an irregularly wavy fine boundary line, which marked the limit of the area, and the outer edges of the peripheral row of cells, the protoplasm of which is denser and therefore darker in the preparations than that of the rest of the shield. The cells next the boundary row are often intermediate in character. Sections of the shield, fig. 1, A, show that the ectoderm consists of high narrow cylinders, between which imperfect cell |