Text-book of comparative anatomy . urrow has become a fur-row round the whole cir-cumference of the eudodermdisc and is thus middle part of the en-doderm disc sinks down, sothat it now becomes a de-pression with a somewhatraised floor (gastrula inva-gination). This depressionrepresents the rudimentarymidgut, and the outer edgesof the original circularfurrow are the edges ofthe blastopore or gastrulamouth. In front of theanterior edge of the blasto-pore lie the primitive meso-derm cells in active growth,and giving off cells towardsthe centre of the blasto-sphere (Fig. 275). Stage D
Text-book of comparative anatomy . urrow has become a fur-row round the whole cir-cumference of the eudodermdisc and is thus middle part of the en-doderm disc sinks down, sothat it now becomes a de-pression with a somewhatraised floor (gastrula inva-gination). This depressionrepresents the rudimentarymidgut, and the outer edgesof the original circularfurrow are the edges ofthe blastopore or gastrulamouth. In front of theanterior edge of the blasto-pore lie the primitive meso-derm cells in active growth,and giving off cells towardsthe centre of the blasto-sphere (Fig. 275). Stage D. The primitivemouth is in the act ofclosing.—In the centres ofthe two cephalic lobes arethe rudiments of the the cephalic lobesand the thoraco-abdominalrudiments the first tracesof the mandibles and an-tennae appear. The primi-tive mouth closes from be-fore backward, its lateraledges growing together inthe middle line. The cellsof the primitive intestinebegin to consume nutritiveyolk. /; I > TACEA—ONTOGENY 401. FIG. 272.—Astacus fluviatilis, embryo in the Nauplius stage.—Stage F. A (above), Eudiment of eye ; I, upper lip; G, cerebralganglion ; «j, anterior antenna;; rjan, ganglion of the segment of the2d antennae (a»); gm, ganglion of the segment of the mandibles (m);Ta, thoraco-abdomiual rudiments; A, (below) anus (after Reichen-bach). Stage E. Embryo,with mandibles begin-ning to form. — In themiddle of the thoraco-abdomiual disc the (ecto-dermal) rudiments of theanus and hind-gut (proc-todteum) appear in theform of an anal mesodermal elementsspread out below theectoderm. Stage F. Embryo inthe Nauplius stage(Figs. 272 and 276).—The 3 most anterior pairsof extremities (Waupliusextremities) can be dis-tinctly made out. Be-tween the anterior an-tenna} the first (paired)traces of the brain appear,and immediately behindthere is a pit-like de-pression, the ectodermalrudiments of the mouthand fore - gnt (storno-dffium). In the posteriorantennal
Size: 1512px × 1653px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
