. Biology . the front end and afterward at theposterior end. The girdle thus forms a cocoon, which hardenslater into a chitinous spindle-shaped vessel containing repro-ductive cells and albuminoid food material (Fig. 65). The eggsare fertilized in the coccoon by the spermatozoa, and develop-ment begins at once, continuing under the protection of thecocoon. Cleavageof the egg is regular up to the i6-cell stage, with fourvegetative cells at the lower, and smaller animal cells at theupper pole. The lower cells invaginate and form a typical two- EMBRYOLOGY OF THE EARTHWORM 159 layered gastrula. Up
. Biology . the front end and afterward at theposterior end. The girdle thus forms a cocoon, which hardenslater into a chitinous spindle-shaped vessel containing repro-ductive cells and albuminoid food material (Fig. 65). The eggsare fertilized in the coccoon by the spermatozoa, and develop-ment begins at once, continuing under the protection of thecocoon. Cleavageof the egg is regular up to the i6-cell stage, with fourvegetative cells at the lower, and smaller animal cells at theupper pole. The lower cells invaginate and form a typical two- EMBRYOLOGY OF THE EARTHWORM 159 layered gastrula. Up to this stage, development closely followsthe type described on page 79, but from here on, it becomescomplicated by the formation of a third germ layer, called themesoderm. This arises from two pole cells, coming from thevegetative pole and taking an initial position in the segmenta-tion cavity (Fig. 66). They then divide, forming a sheath ofcells on each side of the median line and filling the segmentation. c Fig. 65.—A, Egg capsule, enlarged five diameters (a few eggs, ov., are shownnear by on the right enlarged to the same scale); J5, an ovum highly magnified;C, a spermatozoon still more highly magnified; n, nucleus or head; m, middlepiece; and t, tail. (From Sedgwick and Wilson.) cavity. In the meantime, the embryo has elongated in themain or antero-posterior axis passing through the blastopore;new ectoderm cells are pushed in from the ectoderm, and sec-ondary mesodermal pole cells are separated from the former are the first stages of the nervous sytem, and areknown as neuroblasts. The latter are of different kinds, withdifferent functions to play later. Some are muscle-formingcells called somatoblasL, and some are nephridia-forming,known as nephrohlasts. All give rise to sheets of cells which be-come differentiated into the ultimate adult structures—nervoussystem, muscles and nephridia. Meanwhile, the masses of mesoderm cells on each side of themedian lin
Size: 2046px × 1221px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectbiology
