General principles of zoology . des the blastula there is still a secondstage of development, the gastrula or the two-layered em-bryo, which is common to all the stage is understood easiest in thecase of eggs which have an equal cleavage(Fig. 101, B]; here it has the form of adouble-walled cup with a wider or nar-rower mouth. The cavity of the cup (theprimitive digestive tract orarclicntcroii) isthe rudiment of the most important partof the digestive system ; the opening isthe primitive mouth or the prostouia(called also the blastopore). Of the twolayers of cells forming the wall
General principles of zoology . des the blastula there is still a secondstage of development, the gastrula or the two-layered em-bryo, which is common to all the stage is understood easiest in thecase of eggs which have an equal cleavage(Fig. 101, B]; here it has the form of adouble-walled cup with a wider or nar-rower mouth. The cavity of the cup (theprimitive digestive tract orarclicntcroii) isthe rudiment of the most important partof the digestive system ; the opening isthe primitive mouth or the prostouia(called also the blastopore). Of the twolayers of cells forming the wall of the cupand uniting at the prostoma, the exter- FlG 10I7TGastruiation ofnal is the cctoblast or outer gcrin-laycr,the internal the cntoblast or inner germ-layer. In the gastrula stage, we meetfor the first time the formation of germ-layers, , the formation of definiteembryonic layers marked off from eachother, the cells not yet differentiated,from which organs arise through organological and his-tological (AfterHatschek.) The animalpole here is above, andthe vegetal pole below,in comparison with In Fig. A the cellsof the vegetal pole arebeginning to sink in B,the invagination com-pleted ; the cleavage cav-ity reduced to a slit be-tween the entoblast (en)and the ectoblast (ek) • o,gastrula-month. 188 GENERAL PRINCIPLES OF ZOOLOGY. Invagination.—The gastrula is formed from the blastulaby imagination (Fig. 101, A). The result is the same aswhen by pressure of the finger upon a hollow india-rubberball one side is pressed in against the other; the layer ofvegetal cells gradually sinks in and becomes surroundedby the cells of the animal pole (Fig. 101, B}. Thus therearises in the egg, in addition to the cleavage cavity, a newcavity, the rudiment of the lumen of the digestive tract;this increases and finally entirely obliterates the cleavagecavity, so that the invaginated part of the blastoderm, theentoblast, becomes pressed against the part which r
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
