The porifera and coelentera . an adventitious gastral ray has been added. It isremarkable that this fourth ray is derived from a distinct source from theother three, its scleroblast, or [lastral actinoblast, as it may be termed, beingderived from a porocyte at a comparativtly hite period in the growth ofthe basal system. After the three basal rays have reached a certainlength, tlie nucleus of a neiglibouring porocyte divides, and a portion ofthe cell, with one of the nuclei, becomes constricted otf, grows out towardsthe minute triradiate, takes up a position over it— internal to it—andsecr
The porifera and coelentera . an adventitious gastral ray has been added. It isremarkable that this fourth ray is derived from a distinct source from theother three, its scleroblast, or [lastral actinoblast, as it may be termed, beingderived from a porocyte at a comparativtly hite period in the growth ofthe basal system. After the three basal rays have reached a certainlength, tlie nucleus of a neiglibouring porocyte divides, and a portion ofthe cell, with one of the nuclei, becomes constricted otf, grows out towardsthe minute triradiate, takes up a position over it— internal to it—andsecretes a minute spicule ray whicli becomes fused and tacked on to thebasal triradiate system (Fig. 75, 4). The secretion of the gastral raymay commence before its actinoblast is completely separated from theporocyte. In the further development the nucleus of the gastral actino-blast may remain single or divide into two or four nuclei, according tothe size of the ray to be formed. In all cases, however, the protoplasm frsysf. Fin. 75. Devclopinent of equiangular triradiates and qnadriradiatos in Clathrina. 1, trio of actino-blasts ; •_, sextet, with young spicule; 3, late stage in the growth of the spicule, after loss ofthe apical formative cells ; 4, division of a porocyte to form a gastral actinoblast; 0, late stagein the secretion of the gastral ray. , triratliate system ; , basal formative cell; actinoblast; , gastral ray ; p, dermal aperture of pore. of the actinoblast remains undivided, and covers at first the whole ray(Fig. 75, 5), but later only its tip, in the form of a granular Plas-modium, very different in appearance from the formative cells of thebasal system which, at first granular, soon become very clear and freefrom conspicuous granulations. It is evident from their develoi)ment that the many-rayed spi(fules ofClathriniihte, and jJioViably uf all Calcarea, are compound sjiicules, repre-senting a spicular .\ystem derived from
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Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
