EMBRYOLOGY OF THE SPONGE. 



323 



stouter, the nucleus remains a little behind in a shallow depression, marked on. the adult globate as 

 the "hilum." After the trichites have become strong spines, they grow rounded at the ends, then 

 toothed and roughened for the attachment of ligamentous fibres. 



Embryoloyy (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 



Fig. 12. DEVELOPMENT OF A SILICIOUS SPONGE (plafctno monolopfca). [After ScJiulze.] 



A, Free-*wfmmiDg Planula ; u, section of the same, showing epiblast, and hypolilast ; c, fixed planula with a gastric cavity; D, section of tlie same 

 a lit tie older; K, yotine sponqre, showing pores and flagellated chambers : F, section across the adult sponge, showing (sp) sperm ball, (ovt ova 

 in different stages of development, and (bll lilastula. 



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 an Amphi-blastula 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 

 hypoblast 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 incurrent canals of the mother 

 Sponge, and is carried out by the outflowing currents into the surrounding water, when it swims 



