. The physiology of reproduction. Reproduction. 464 THE PHYSIOLOGY OF REPRODUCTION the embryonic circulation has found the shortest route towards the trophoblast by way of the 'ventral stalk,' trophoblastic lacunae, with their profusion of maternal blood, which have been there from the very earhest periods of development, are exquisitely situated for rendering this new adaptation highly advantageous. And while in the ancestral forms of the Primates both yolk-sac and allantois largely drew upon the trophoblastic source, these embryonic organs come to be dispensed with to a very great extent in
. The physiology of reproduction. Reproduction. 464 THE PHYSIOLOGY OF REPRODUCTION the embryonic circulation has found the shortest route towards the trophoblast by way of the 'ventral stalk,' trophoblastic lacunae, with their profusion of maternal blood, which have been there from the very earhest periods of development, are exquisitely situated for rendering this new adaptation highly advantageous. And while in the ancestral forms of the Primates both yolk-sac and allantois largely drew upon the trophoblastic source, these embryonic organs come to be dispensed with to a very great extent in their more highly developed descendants who come to use that trophoblastic source along a more direct, a shorter, and an earher estabhshed ;. Fig. 120.—Median longitudinal section of an early human ovum, 0"4 mm. in length. (From Quain's Anatomy, Longmans.) , embryonic ectoderm ; ch., chorion ; ec, ectoderm ; mes., mesoderm ; all., allantois ; , connecting stalk ; a., amnion ; , yolk-sac. In old-world monkeys there is no decidua capsularis. The trophoblast thickens over two discoid areas on the blastocyst, and the thickenings form a primary placenta on the dorsal surface, and a smaller secondary placenta on the opposite aspect. Hence two groups of chorionic villi are developed. No unattached blastocyst has yet been obtained. In the youngest specimen of an old-world monkey, Semnopithecus. nasicus, the ovum was attached to the surface of the uterus by large villous processes with mesoblastic cores at the bases. The trophoblast consisted of two layers, the cytoblast, which was much thickened at the tips of the mesoblastic cores, and, externally to it, a syncytium which was blended at the apices with maternal decidua. Over the non-viUous chorion, syncytium. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resembl
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectreproduction, bookyea
