. The biology of the frog . ally a hollow sphereof which the wall on the vegetative side is very much thickerthan it is above and composed of large yolk-laden cells. Gastrulation. — In the embryonic development of most ofthe many-celled animals a stage is passed through which isknown as the gastrula. In its typical form a gastrula is a sortof double-walled sac such as may be produced, according toa well-worn illustration, by pushing in one side of a hollowrubber ball with the finger. The mouth of the gastrula iscalled the blastopore, and this opening naturally becomes 04 THE BIOLOGY OF THE FRO
. The biology of the frog . ally a hollow sphereof which the wall on the vegetative side is very much thickerthan it is above and composed of large yolk-laden cells. Gastrulation. — In the embryonic development of most ofthe many-celled animals a stage is passed through which isknown as the gastrula. In its typical form a gastrula is a sortof double-walled sac such as may be produced, according toa well-worn illustration, by pushing in one side of a hollowrubber ball with the finger. The mouth of the gastrula iscalled the blastopore, and this opening naturally becomes 04 THE BIOLOGY OF THE FROG CHAP. smaller as the process of in-pushing is completed. The pro-cess of gastrulation, which is exemplified in its typical form inthe development of a starfish or sea-urchin, becomes verymuch modified in different animals. Such is the case in thedevelopment of the frog. The large accumulation of yolk atthe vegetal side of the blastula prevents the invagination ofthis region from taking place in the typical way. The same. FlG. 19. — Sagittal section through a frog embryo. B, blastoccel or segmen-tation cavity; DP, lip of blastopore; EE, outer or epidermic layer ofectoderm; EN, inner or nervous layer of ectoderm; Y, yolk cells.(After Marshall.) end is reached partly by a process of in-pushing and partlyby the overgrowth of the white pole by the dark. The in-pushing and overgrowth take place more on one side of theegg than the other, and these processes are first indicatedby the appearance of a crescentic groove a little below theequator of the egg. The crescent represents the beginningof the blastopore. The groove is deepest at the center and V THE DEVELOPMENT OF THE FROG 95 thins out toward the edges, which gradually extend aroundthe lower pole of the egg. In this way the crescent becomesconverted into a circle, and the circle gradually becomessmaller and smaller until only a small part of the light-coloredyolk, known as the yolk plug, appears in the midst of the darkarea.
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