A system of obstetrics . Rabbits Ovum between Seventy and Ninety Hours after Impregnation (after E. van Beneden):bv, cavity of blastodermic vesicle or yelk-sac; ep, epiblast; hy, primitive hypoblast; Zp,layer formed by zona pellucida and swollen and degenerated cells of discus proligerus. Although by this stage, which occurs in the rabbit between seventyand ninety hours after impregnation, the blastodermic vesicle has by nomeans attained its greatest dimensions, it has nevertheless grown fromabout mm. (-^ inch)—the size of the morula at the close of seg-mentation—to about mm. (^ inch
A system of obstetrics . Rabbits Ovum between Seventy and Ninety Hours after Impregnation (after E. van Beneden):bv, cavity of blastodermic vesicle or yelk-sac; ep, epiblast; hy, primitive hypoblast; Zp,layer formed by zona pellucida and swollen and degenerated cells of discus proligerus. Although by this stage, which occurs in the rabbit between seventyand ninety hours after impregnation, the blastodermic vesicle has by nomeans attained its greatest dimensions, it has nevertheless grown fromabout mm. (-^ inch)—the size of the morula at the close of seg-mentation—to about mm. (^ inch) in diameter. It is enclosed bya membrane formed from the zona radiata and the mucous layer aroundit. The blastodermic vesicle continues to enlarge rapidly, and during theprocess the hvpoblastic mass undergoes important changes. It spreadsout on the inner side of the epiblast, and at the same time loses its lens-. 104 DEVELOPMENT OF THE EMBRYO. like form ;m<I becomes flattened. The central part of it, however, thick-ens, an<l becomes constituted of two rows of cells, while the peripheralpart, the outer boundary of which is irregular, is formed of an imper-fect layer of amoeboid cells, which continually spread farther and fartherwithin the epiblast. The central thickening of the hypoblast forms anopaque circular spot on the blastoderm, \\ hich constitutes the commence-ment of the embryonic area The structure of the developing ovum at this stage is diagrammati-cally represented in Fig. 18. It is three layers thick in the embryonic area. The outer cells at this region, derived^1G-^8, from the primitive epiblast layer, are known as the cover-cells. They do not directly sharein the formation of the embryo. The rest ofthe primitive epiblast (non-embryonicepiblast,epf) helps to form the amnion and umbilicalvesicle. (See p. 126.) The second laver ofcells in the embryonic layer, , becomes
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
