Volume 1

Quain's elements of anatomy / [Jones Quain].

  • Jones Quain
Date:
1890-1896
Catalogue details

Licence: Public Domain Mark

Credit: Quain's elements of anatomy / [Jones Quain]. Source: Wellcome Collection.

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    sixteen clear cells and only eight darker cells, for the same reason. Farther, it is found that as the segmentation proceeds the clearer cells occupy the superficial part of the ovum and almost entirely enclose the granular cells which fill the interior (fig. 14, a). The ovum next undergoes a rapid increase in size owing to the segregation of fluid between the clear superficial layer of cells and the enclosed granular segments, which thus become separated from one another except at one part (fig. 14, &). At the same time the superficial cells multiply, and, becoming flattened out like a pavement epithelium, form a membrane enclosing the contained fluid. The ovum is now a thin-walled transparent sac, occupied by fluid and enclosed by two membranes, one Fig. 14.—Sections op the ovum op the rabbit DURING THE LATER STAGES OF SEGMENTATION, SHOWING THE FORMATION OP THE BLASTODER¬ MIC vesicle. (E. v. Beneden.) a, Section showing the enclosure of darker cells, ent, by clearer cells, ect; b, more advanced stage in which fluid is beginning to accumulate be¬ tween the inner and outer cells, the former com¬ pletely enclosed ; c, the fluid has much increased, so that a large space separates inner from outer cells except at one part ; d, blastodermic vesicle, its wall formed of a layer of flattened cells, with a patch of dark granular cells adhering to it at one part ; z.p., zona pellucida. being the thinned-out zona pellucida and the other the epithelial membrane just mentioned. Adherent to one part of the inner surface of this membrane is the little mass of dark granular cells which formerly occupied the whole interior of the mulberry mass, and these cells give to the part of the ovum where they occur a darker appearance, when it is viewed by transmitted light. At this stage of development the ovum has been termed the blastodermic vesicle (fig. 14, c, d), although the actual blastoderm is not yet formed. Formation of the blastoderm.—Soon the granular cells are found to be no longer accumulated into a small mass but to be spreading out in the form of a lenticular patch over the inner surface of the vesicle. As this extension proceeds the innermost cells separate off as a distinct layer, the separation starting from the centre and progressing outwards. A section through the middle of the ovum now shows three layers (fig. 15) : an outer, which is the epithelial membrane of the blastodermic vesicle (Rauber’s layer); an inner, which may be termed the primitive entoderm, from the fact that it becomes the innermost layer of the blastoderm, and an ill-defined middle stratum of somewhat VOL. i. c
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