. Cassell's natural history. Animals; Animal behavior. EMBRYO Lour OF THE SPONGE. stouter, tlie nucleus remains a little behind in a shallow depression, marked on the adult globate as the "; After the trichites have become strong spines, they grow rounded at the ends, then toothed and roughened for the attachment of ligamentous fibres. Embrijohfjy (Figs. 12, 13).—Notwithstanding the attention which has been paid to the embry- ology of the Sponges, it is still impossible to bring our knowledge on the subject under a single large. ZJ"" t Btagtsof (ltvilui>iu) siierm
. Cassell's natural history. Animals; Animal behavior. EMBRYO Lour OF THE SPONGE. stouter, tlie nucleus remains a little behind in a shallow depression, marked on the adult globate as the "; After the trichites have become strong spines, they grow rounded at the ends, then toothed and roughened for the attachment of ligamentous fibres. Embrijohfjy (Figs. 12, 13).—Notwithstanding the attention which has been paid to the embry- ology of the Sponges, it is still impossible to bring our knowledge on the subject under a single large. ZJ"" t Btagtsof (ltvilui>iu) siierm ball, ( generalisation. Two distinct modes of development have been so far fairly made out, but it yet remains to be seen how far all Sponges conform to these, and how they are related to each other. The ovum in all cases divides first into two, then four, and next eight segments, which, however, are not always equal and similar ; by further subdivision, it gives rise either to a solid cluster of cells like a mulberry (morula), or a hollow spherical cluster, the cells forming a single layer about a central cavity (blastula), which normally is completely closed, but, in one instance at least, is known to be open at the poles. The course of development may now become very different, according as a Planula or &n AmpM-hlmttda is next formed. The planula is a solid embryo (Fig. 12, a) of two layers of cells—an inner, or hypoblast, consisting usually of gelatinous connective tissue, with its stellate corpuscles, and an outer, or epiblast, consisting of small, cylindrical, flagellated cells. The hypoljlast originates either by metamorphosis of the internal cells of the morula, or by the budding of fresh cells, which subsequently become metamorphosed, from the inner ends of the cylindrical cells of the blastula. The planula at this stage usually escapes into one of the incurreut canals of the mother Sponge, and is carried out by the outflowing currents into the surrounding wate
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Keywords: ., bookcentury1800, bookdecade1890, booksubjecta, booksubjectanimals
