. Outlines of zoology. Zoology. DEVELOPMENT OF SPONGES. 123 history of the germinal layers in Sponges into line with that in other Metazoa. (a) In the small calcareous sponge Sycandra raphanus, as described by F. E. Schuize, the segmentation results in a hollow ball of cells—the blastida. A few cells at the lower pole remain large, and are filled with nutritive granules ; the other cells divide rapidly and become small, clear, columnar, and flagellate. The large granular cells become temporarily invaginatecl, form- ing what is called a ^^pseiido- gastrzila.^^ This leaves the parent and the gra
. Outlines of zoology. Zoology. DEVELOPMENT OF SPONGES. 123 history of the germinal layers in Sponges into line with that in other Metazoa. (a) In the small calcareous sponge Sycandra raphanus, as described by F. E. Schuize, the segmentation results in a hollow ball of cells—the blastida. A few cells at the lower pole remain large, and are filled with nutritive granules ; the other cells divide rapidly and become small, clear, columnar, and flagellate. The large granular cells become temporarily invaginatecl, form- ing what is called a ^^pseiido- gastrzila.^^ This leaves the parent and the granular cells right themselves, forming the posterior hemisphere of the embryo, now called an aiiiphi- blasiula. It swims for a time actively, but the flagellate cells of the anterior hemisphere are invaginatecl into or overgrown by the large granular cells, and thus what is generally called the gastrula stage results. This soon settles down, on rock or seaweed, with the blastopore or gastrula mouth downwards, and is moored by amceboid pro- cesses from the granular cells, which likewise obliterate the blastopore. The granular cells lose their granules, for the larva is not yet feeding ; the now in- ternal flagella disappear in the absence of the stimulating water; a mesoglcea with spicules begins to be formed between the inner Fig. 34.—Development of Sycandra raphanus. (After F. E. ) 1. Ovum. 2. Section of 16 cell .stage. 3. Blastula with S granular cells ij^) at lower pole. 4. Free swimming amphiblastula, with upper hemisphere of flagellate cells (f-c), and lower hemisphere of granu- lar cells. 5. stage settled down. Ec., outer layer (ectoderm ? ) ; £n., inner layer(endoderm ? ); bL, closing blastopore; , utooring amceboid Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the orig
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
