. Outlines of zoology. where the em-bryo begins to be the other end, be-tween the blastoderm andthe yolk, lies a segmentationcavity (Fig. 290, ). At, the embryonic endthe outer layer or epiblastundergoes a slight invagina-tion (Fig. 290, .X.), beginningto form the roof of thefuture gut (g.); in otherwords, establishing the hypo-blast. This inflected arc ofthe blastoderm correspondsto the blastopore or mouthof the gastrula, which is-much disguised by the pres-ence of a large quantity ofyolk. As the invaginationproceeds, the segmentationcavity is obliterated. Thefloor of the g
. Outlines of zoology. where the em-bryo begins to be the other end, be-tween the blastoderm andthe yolk, lies a segmentationcavity (Fig. 290, ). At, the embryonic endthe outer layer or epiblastundergoes a slight invagina-tion (Fig. 290, .X.), beginningto form the roof of thefuture gut (g.); in otherwords, establishing the hypo-blast. This inflected arc ofthe blastoderm correspondsto the blastopore or mouthof the gastrula, which is-much disguised by the pres-ence of a large quantity ofyolk. As the invaginationproceeds, the segmentationcavity is obliterated. Thefloor of the gut is formed byinfolding of the lateral walls. Along the mid-dorsal line of the epiblast a medullarygroove appears â the beginning of the central nervoussystem. Its sides afterwards arch towards one another, andmeet to form a medullary canal (Fig. 290, ). A posteriorcommunication between this dorsal nervous tube above andthe ventral alimentary tube persists for some time as theneurenteric canal (Fig. 290, ).. Fig. 291.âEmbryo dogfish in egg-case (mermaids purse) whichhas been cut open to show con-tents. , External gills; , dorsal finfold ; j/.r., yolk-sac ; si., stalk of yolk-sac ; T., tendrils, prolongations of egg-case by means of which it is moored toseaweed, an adaptation in a deadsubstance. SSo PISCESâFISHES. The mesoblast arises as two lateral plates, one on eachside of the medullary groove. The plates seem to arise asa pair of solid outgrowths from the wall of the gut. Theyare afterwards divided into segments. Between the meso-blast plates, along the mid-dorsal line of the gut, thenotochord is established (Fig. 290, n.). Besides the internal establishment and differentiation oflayers, there are two important processes,â{a) the growthof the blastoderm around the yolk, (/) the folding off ofthe embryo from the yolk. The result of the two processesis that the yolk is enclosed in a yolk-sac, with which theembryo is finally connected only by a thin stalkât
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
