Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 PKIMITIVE STREAK. 115 of the gastrula has hitherto only been observed in some hydroid Medusse (Geryonia). Finally, when the inequality of the segmentation is very pro- nounced, the gastrula is formed by a process known as epibole. In this process of development the epiblast cells, which are early distin- guishable from the much larger hypoblast cells, spread themselves over the latter as a thin layer (fig. 110); and in this, as in the second method of development of the gastrula, the cav
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 PKIMITIVE STREAK. 115 of the gastrula has hitherto only been observed in some hydroid Medusse (Geryonia). Finally, when the inequality of the segmentation is very pro- nounced, the gastrula is formed by a process known as epibole. In this process of development the epiblast cells, which are early distin- guishable from the much larger hypoblast cells, spread themselves over the latter as a thin layer (fig. 110); and in this, as in the second method of development of the gastrula, the cavity of the gastrula is, as a rule, a secondary formation in the centre of the closely-packed mass of hypoblast cells. The blastopore is usually found at the point where the complete enclosure of the hypoblast is effected. It sometimes happens that a part of the primary blastosphere is developed more rapidly than the remainder, and gives rise to a A
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