. Biology; the story of living things. ...mesooCarm. mesoderm onus V. segments 9 and 10, it receives sperm cells from the other worm that have been stored there. The girdle is passed down over the anterior end of the worm, slipped off, forming a closed case which contains the eggs, sperms, and a nutritive fluid. These capsules may be found in late spring under stones, boards, logs, or in manure heaps. After fertilization, the egg of the earthworm divides first into two, then four, then eight cells, and so on, continuing until a hollow ball of cells, called a hlastula, is formed. These cells ar
. Biology; the story of living things. ...mesooCarm. mesoderm onus V. segments 9 and 10, it receives sperm cells from the other worm that have been stored there. The girdle is passed down over the anterior end of the worm, slipped off, forming a closed case which contains the eggs, sperms, and a nutritive fluid. These capsules may be found in late spring under stones, boards, logs, or in manure heaps. After fertilization, the egg of the earthworm divides first into two, then four, then eight cells, and so on, continuing until a hollow ball of cells, called a hlastula, is formed. These cells are not all the same size, larger cells appearing on the lower pole of the sphere, which begins to flatten and show a depres- sion, forming eventually a hollow cuplike affair, called the gastrula. This process known as gastrula- tion places the larger cells of the lower pole on the in- side of the cup where they become the endoderm, leaving the outer cells of the sphere to form the ectoderm. Meantime a third layer of cells which lies between the other two layers buds off and be- comes the mesoderm. This latter layer gives rise to the musculature, blood vessels, and most of the excretory and reproductive tissues ; the endoderm forms the food tube and much of the glandular material con- nected with it; the ectoderm gives rise to the epiderms, the nervous system and sense organs, and the outer portions of the nephridia, repro- ductive ducts, and digestive tracts. The young worms remain in th(^ egg case until they are about an inch in length. When first hatched they have no clitellum, since this organ appears only in mature gostrola Stages in development of earthworm. Fig- ures II-V. Segmentation of egg and formation of blastula. Figures VI-VIII. Sections, show- ing formation of mesoderm as a band of cells. IX. Late stage of gastrula, showing coelomic spaces in mesoderm bands. X. Longitudinal section of young worm showing food tube, mouth and anus. (After Sedgwick and Wilson.
Size: 1933px × 1293px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorwalterherberteugeneb1867, bookcentury1900, bookpublish
