Outlines of zoology . on borne to it by the exhibits a certain power ofmigration, as in some to fertilisation, the usualextrusion of polar bodies hasbeen observed in a few cases,and is doubtless general. Seg-mentation is total and usuallyequal, and results in a sphericalor oval embryo more or lessflagellate. This leaves the parentsponge, swims about for a time,then settles down, and undergoesa larval metamorphosis oftendifificult to understand. It ispeculiarly difficult to bring thehistory of the germinal layers insponges into line with that inother Metazoa. Fig. 61.
Outlines of zoology . on borne to it by the exhibits a certain power ofmigration, as in some to fertilisation, the usualextrusion of polar bodies hasbeen observed in a few cases,and is doubtless general. Seg-mentation is total and usuallyequal, and results in a sphericalor oval embryo more or lessflagellate. This leaves the parentsponge, swims about for a time,then settles down, and undergoesa larval metamorphosis oftendifificult to understand. It ispeculiarly difficult to bring thehistory of the germinal layers insponges into line with that inother Metazoa. Fig. 61.—Development of ^y^flKisfr-fl:raphanus. —After F. E. Schulze. 1. Ovum. 2. Section of i6-cell stage. 3. Blastula with 8 granular cells {^ at lower pole. 4. Free-swimming amphiblastula, with? upper hemisphere of flagellate cells (), and lower hemisphere of granu-lar cells (£). 5. Gastrula stage settled down, ^c, outer layer; £n., inner layer; ii., closingblastopore; , mooring, 132 PHYLUM PORIFERA SPONGES. (a) In the small calcareous sponge Sycandra raphanus (Fig. 6l), asdescribed by F. E. Schulze, the segmentation results in a hollow ballof cells—the blashila. A few cells at the lower pole remain large,and are filled with nutritive granules; the other cells divide rapidlyand become small, clear, columnar, and flagellate. The large granularcells become invaginated, forming what is called a leaves the parent, and the cells forming the lower hemisphereof the embryo become rounded andnon-flagellate. The embryo swims fora time actively, but the flagellate cellsof the upper hemisphere are invaginatedinto or overgrown by the large granularcells, and thus what is generally calledthe gastrula stage results. This soonsettles down, on rock or seaweed, withthe blastopore or gastrula mouth down-wards, and is moored by amoeboidprocesses from the granular cells, whichlikewise obliterate the blastopore. Thegranular cells lose
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
