Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 268 ELEiVIENTS OF BIOLOGY the floor is formed by the large yolk cells of the vegetal hemi- sphere. The pigmented cells of the animal hemisphere increase in number and migrate downward over the vegetal hemisphere, thus incorporating the yolk-containing vegetal hemisphere cells within as the floor of the archenteron. The blastopore is then first indicated by the crescentic shaped notch where the endoderm is first turned inward. This is the dorsal
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 268 ELEiVIENTS OF BIOLOGY the floor is formed by the large yolk cells of the vegetal hemi- sphere. The pigmented cells of the animal hemisphere increase in number and migrate downward over the vegetal hemisphere, thus incorporating the yolk-containing vegetal hemisphere cells within as the floor of the archenteron. The blastopore is then first indicated by the crescentic shaped notch where the endoderm is first turned inward. This is the dorsal lip of the blastopore, the lateral ANIMAL HEMISPHERE BLASTOCOELE ARCHENTERON MESODERM DORSAL LIP OF BLASTOPORE VOLK CELLS VEGETAL HEMISPHERE Fig. 182.—Section through the gastrula stage of the frog embryo. and ventral lips being the line formed where the animal hemis- phere cells are advancing over the vegetal hemisphere to converge and form a circular blastopore under the dorsal lip. In the distinctly teleolecithal eggs of fishes, reptiles, and birds the yolk-containing vegetal hemisphere does not cleave. Cell divisions are at first confined to a region in the vicinity of the animal pole, the cells forming a cap. Gastrulation is much modified. In birds it is reduced to the inward growth of a wedge of cells, which forms the endoderm by increasing in area by cell divisions and by addi- tions of cells cleaved from nucleated cytoplasm that underlies the
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