Text-book of comparative anatomy . the thin layer of protoplasm which is found at the surface of thegerm. They feed at the expense of the deutoplasm. The 8 merocytes divide into 16, 32, and so on, and move at the same timecentrifugally through the yolk to the surface of the germ, where they form a simplecontinuous layer. It is this layer of merocytes which is called the germ has now become centrolecithal, and agrees in its structure with the Geryonid II SEGMENTATION AND GASTRULATION 127 germ before the formation of micromeres or delamination, except that the centralnutritive yol
Text-book of comparative anatomy . the thin layer of protoplasm which is found at the surface of thegerm. They feed at the expense of the deutoplasm. The 8 merocytes divide into 16, 32, and so on, and move at the same timecentrifugally through the yolk to the surface of the germ, where they form a simplecontinuous layer. It is this layer of merocytes which is called the germ has now become centrolecithal, and agrees in its structure with the Geryonid II SEGMENTATION AND GASTRULATION 127 germ before the formation of micromeres or delamination, except that the centralnutritive yolk is here still undivided. The further development of the germ has unfortunately not yet been thoroughlyinvestigated. Some observations tend to show that at this point, as in the Geryonidgerm, delamination or formation of micromeres takes place. The nierocytes at thesurface divide in such a way that the outer portion separates off completely as anucleated micromere layer detached on all sides, while the inner portion remains in J. FIG. 97.—Segmentation and formation of blastoderm in the egg of an Insect. the nutritive yolk as a nucleated merocyte layer. Thus the two germ layers areformed ; the outer micromere layer represents the ectoderm ; the central mass ofnutritive yolk with the merocytes which belong to it probably represents theendoderm. The merocytes by increasing in number and feeding at the expense of thenutritive yolk, become able at last to overpower and incorporate it, they candivide it. The gastrula is here a solid sterroplanula. It has been observed in many cases that in the partial segmentation, in themultiplication of the central merocytes, onlysome of them move towards the surface, thereto form the blastoderm, while the othersremain in the yolk. 5. The meroblastic telolecithal germ (). We left this germ at the 8-blastomerestage, with undivided mass of mitritive may here also call the masses of proto-plasm merocytes. They stand in exact
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
