Lectures on the early stages in the development of mammalian ova and on the formation of the placenta in different groups of mammals / by Arthur Robinson.

  • Robinson, Arthur.
Date:
[1903]
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Licence: In copyright

Credit: Lectures on the early stages in the development of mammalian ova and on the formation of the placenta in different groups of mammals / by Arthur Robinson. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    In the majority of cases the trophoblast is situated entirely on the surface of the blastocyst, but in the rat, mouse, and guinea-pig a portion of it is located in the inner cell mass in the interior of the ovum, from which it is subsequently differentiated. Obviously, therefore, mere position at the end of segmentation is no absolute indication of the future fate or functions of a cell, except in the cases of the cells of the external trophoblast, for in some mammals the inner mass is the rudiment of the entoderm and the embry- onic ectoderm alone, in others it also includes the rudiment of the amniotic ectoderm, and in the rat, mouse and guinea-pig, it con- tains, in addition, the rudiment of a portion of the trophoblast, which may be termed the internal trophoblast. It is interesting to note that the internal trophoblast of rodents retains its cellular character, whilst the external trophoblast of rodents forms a nucleated syncytium, as in the hedgehog. The cellular character is retained until a comparatively late stage of development, and the internal trophoblast, therefore, resembles the cytoblast or inner layer of the trophoblast of bats, whilst the external tropho- blast of guinea-pigs, rats, and mice corresponds with the plasmodi- blast of the bats. Compare figs. 17, 18, 19 and 20 with figs 21, 22 and 23. EXPLANATION OF PLATES. B. Body of Balbiani. C. Cytoblast. EAE. Embryo-amniotic ectoderm. EN. Entoderm. IM. Inner cell mass. IT. Internal trophoblast. M. Membrana granulosa. MM. Mesoderm. P. Plasmodiblast. S. Spaces which become the cavity of the Graffian follicle. T. Trophoblast. Z. Zona pellucida. Fig. 1. Section of the ovum of a ferret surrounded by a single layer of membrana granulosa cells. The zona pellucida has begun to appear, x 190. Fig. 2. Section of a ferret’s ovum when the membrana granulosa possesses in some places two, and in others three, rows of nuclei. The zona pellucida is very little thicker than in the preceding stage, but the bodies of some of the surround- ing membrana cells have become fibrillar and their inner ends are fused with the outer surface of the zona pellucida. x 190. Fig. 3. Section of a ferret’s ovum when the surrounding membrana granulosa contains five rows of nuclei, and the rudiments of the cavity of the Graffian follicle have begun to appear as small fluid-filled spaces between the bodies of some of the cells. The zona pellucida is now distinctly thicker than in the preceding stages, and it is still closely connected with the inner extremities of some of the cells of the membrana granulosa, x 185. Fig. 4. Section of a ferret’s ovum which lay in an almost mature Graffian follicle. The zona pellucida is much thicker than in the preceding stages, and it presents a laminated appearance. Its outer surface is closely connected with the cells of the corona radiata, some of which are represented in the upper and left part of the figure, x 185. Fig. 5. Section of a segmenting ovum of a ferret from the upper end of the uterus. The zona pellucida is thicker than in any ovum still lying in the ovary, and after staining and hardening it had become brittle, and was fragmented by the cutting razor. There was no trace of an albumen layer upon it. x 185. Fig. 6. Section of a ferret’s ovum showing the body of Balbiani. x 400. Fig. 7. Section of a portion of the ovary of a half-grown mouse. The membrana granulosa in some of the follicles contains from three to five rows of nuclei, but there is no distinct evidence of a zona pellucida. The dark lines in