The royal natural history . DEVELOPMENT OF A SEA-URCHIN (stage 9). DEVELOPMENT OF A SEA-URCHIN vStageS 1-8). of the embryo, near the number 3, is a smalltuft of cilia, by the motion of which the embryoswims about. In stage 4 this ciliated area isseen to have extended downwards to the letter intestine now develops in such a way thatthe original opening («) remains as the vent, themiddle part (d) widens into a stomach, and afresh mouth-opening (m) is pierced through atits upper end. This is seen from the side instage 4, and from the front in stage 5. Butbefore the mouth is formed, two ear-
The royal natural history . DEVELOPMENT OF A SEA-URCHIN (stage 9). DEVELOPMENT OF A SEA-URCHIN vStageS 1-8). of the embryo, near the number 3, is a smalltuft of cilia, by the motion of which the embryoswims about. In stage 4 this ciliated area isseen to have extended downwards to the letter intestine now develops in such a way thatthe original opening («) remains as the vent, themiddle part (d) widens into a stomach, and afresh mouth-opening (m) is pierced through atits upper end. This is seen from the side instage 4, and from the front in stage 5. Butbefore the mouth is formed, two ear-like pro-cesses (w) show themselves, which are importantas bein the beginnings of the ambulacral andwater-vascular systems. There also appear afew delicate, symmetrically laced rods of car-bonate of lime, which by and by grow into theskeleton of the larva, in shape something likean inverted easel. The two lower ciliated DE VEL OPMENT. 1 ? bands now grow towards one another, so that the vent comes to lie beneaththem (stages
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
