Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 PUIMITIVE STREATC. 115 of the gastnila has hitherto only been observed in some hydroid Medusas (Geryonia). finally, when the ineiniahty of the segmentation is very pi'O- 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
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 PUIMITIVE STREATC. 115 of the gastnila has hitherto only been observed in some hydroid Medusas (Geryonia). finally, when the ineiniahty of the segmentation is very pi'O- 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 s^econd 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 primai-y blastosphere is developed more rapidly than the remainder, and gives rise to a Fig. 110.—^, Unequal segmentation of the egg of Buuellia; B, epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streah is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an erolutio ex una imrte, and the other an evolutio ex omnibus partibus It is not, however, possible to draw a sharp line between these two methods of development, nor haA^e they the significance which was formerly ascribed to them, for closely allied forms may present great differences in this respect according to the amount of food yolk and the duration of the embryonic development. The Coelenterata, the Echinoderms, the lower Worms and Mol- luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) present us with examples of i-egular
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