. A manual of zoology. Zoology. 146 GENERAL PRINCirLES OF ZOOLOGY narro\Yer mouth. The cavUy of the cup (the priniitivo digestive tract or circhfiilcro)!) is the beginning of the most important part of the digesti\e system; the opening is the primitive mouth or blastopore. Of the two layers of cells forming the wall of the cup and muting at the blastopore, the external is the ccloih-rm or outer germ-layer, the internal the entoderm or inner germdayer. Li the gastrula we meet for the lirst time the formation of germdayers, , the formation of defmite embryonic layers marked off from each oth
. A manual of zoology. Zoology. 146 GENERAL PRINCirLES OF ZOOLOGY narro\Yer mouth. The cavUy of the cup (the priniitivo digestive tract or circhfiilcro)!) is the beginning of the most important part of the digesti\e system; the opening is the primitive mouth or blastopore. Of the two layers of cells forming the wall of the cup and muting at the blastopore, the external is the ccloih-rm or outer germ-layer, the internal the entoderm or inner germdayer. Li the gastrula we meet for the lirst time the formation of germdayers, , the formation of defmite embryonic layers marked off from each other, from ^^lâ uch organs arise by differentiation. Invagination.âThe gastrula is formed from the blastula by iiivaginatio)! (lig. 107, A). The result is the same as when one siile of a hollow rulilter ball is pressed into the other; the region of vegetative cells gradually sinks in and Ijccomes sur- rounded by the cells of the animal pole (lig. 107, B). Thus there arises, in addition to the cleavage ^ ^ , cavity, a new cavity, the anlage of the lumen of the Fig 107.âGastrula- - ' - '',-â,,â tion of A mpli digestive tract; tJiis increases and tmally obliterates (after Haischck). The ^j^g cleavage cavity, SO tliat the invaginated part animal pole above, the ^ ^ ' of the blastoderm, the entoderm, becomes pressed against that wltich remains external, the vegetative below, in comparison with lig. loi. In .t the cells of the vegetative pole are beginning to sink in; B, the invagination completed, the cleav- age cavity reduced to a slit between the en to- Modified Modes of Gastrulation.âIn the case of eggs with much food-yolk the relation of the structure and of the mode of formation of the gastrula is more dillicult to understand. Here, however, it is sufticient to say that the gastrula stage has been discovered in derm (en) and iheecto- almost all eggs with a great quantity of food-volk, and derm(.(i);<., blastopore, that the yoik-materialfinds lodgenient
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
