The development of the human body; a manual of human embryology . lt (Fig. 262). By the formation of the fissure the original cavity of the distalportion of the stalk becomesobliterated, and at the sametime the ventral and posteriorwalls of the stalk are broughtinto continuity with the retinallayer of the optic cup, and soopportunity is given for thepassage of the axis-cylinders ofthe ganglion cells along thosewalls (Fig. 260). At an earlystage a section of the proximalportion of the optic stalk (, A) shows the central cavitysurrounded by a number of nu-clei representing the mantlelayer
The development of the human body; a manual of human embryology . lt (Fig. 262). By the formation of the fissure the original cavity of the distalportion of the stalk becomesobliterated, and at the sametime the ventral and posteriorwalls of the stalk are broughtinto continuity with the retinallayer of the optic cup, and soopportunity is given for thepassage of the axis-cylinders ofthe ganglion cells along thosewalls (Fig. 260). At an earlystage a section of the proximalportion of the optic stalk (, A) shows the central cavitysurrounded by a number of nu-clei representing the mantlelayer, and surrounding these anon-nucleated layer resembling the marginal velumand continuous distally with the similar layer of theretina. When the ganglion cells of the latter begin tosend out their axis-cylinder processes, these pass into theretinal marginal velum and converge in this layer towardthe bottom of the ciliary fissure, so reaching the ventralwall of the optic stalk, in the velum of which they may bedistinguished in rat embryos of 4 mm., and still more. Ah Fig. 260. — DiagrammaticLongitudinal Sectionof the Optic Cup andStalk passing throughthe Chorioid Fissure. Ah, Hyaloid artery; L, lens;On, fibers of the opticnerve; Os, optic stalk; PI,pigment layer, and R, retinal layer of the retina. THE OPTIC NERVE. 491 clearly in those of 9 mm. (Fig. 261, A). Later, as thefibers become more numerous, they gradually invade thelateral and finally the dorsal walls of the stalk, and, at thesame time the mantle cells of the stalk become more scat-tered and assume the form of connective-tissue (neurog-lia) cells, while the original cavity of the stalk is gradu-ally obliterated (Fig. 261, B). Finally, the stalk becomesa solid mass of nerve-fibers, among which the alteredmantle cells are scattered. /
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
