so 



SPONGES 



nucleus as a minute s{)lierule easily distinguished by its staining 

 properties from the ordinary cell granules. The spherule grows in 

 length and becomes a rod. The rods of neighbouring cells in each 

 string unite to form a jointed fibre, each segment being separated 

 from the next by an intervening substance less resistent to acids 

 and alkalis (Fig. 51, B). The secreting cells next become spindle- 

 shaped, and their contained rods become in consequence elongated 

 and drawn out (Fig. 51, C and D). At the same time, their 

 substance acquires a denser consistence, more tenacious and less 

 soft The result is a slender fibril, in which the segmentation 

 gradually ceases to be visible, enclosed in a protoplasmic sheath. 

 During this process the secreting cells gradually lose their 



Fl<i. ol. 



l)iaj;rainiiiatic representation of the fonnation of elastic tibrillae in the interior of sponpo- 

 bla.sts (i>orocyte.s ?), after Loisel. A, sjxmgoblasts, each containing a minute, rod-like body, 

 dispostxi invjinlarly at n, arranj;e<l in a row at '<. 7>. the rods are uniting end to end to form 

 a jointed tibril. (', later stage, the rods more elongate, and the cells now almost free from 

 spherules. /), tibril continuous, cells coMinienciiig to degenerate. K^ fully formeii tibril, with 

 adherent cell remnants ; n, luiclei. 



spherules until they are left with a clear cytoplasm and nucleus 

 (Fig. 51, £■). Finally, the fibrils come to lie free in the parenchyma, 

 losing their enveloping cells, the nuclei of which appear to become 

 scattered in the ground substance. The whole process of fibril 

 formation is thus comparable to the secretion of the spicules, each 

 joint being formed in precisely the same manner as a single monaxon 

 spicule, while the whole fibril represents a number of spongin spicules 

 joined end to end, just as a triradiute calcareous spicule represents 

 a system of monaxons joinetl at a centre. On the other hand, the 

 secretion of these fiV)rils ap[)ears to be in every way comparaViIe 

 to the secretion by the porocytes of a fibrillar framework in the 

 gastral cavity of many Calcarea. It is therefore highly probable 

 that the cdliilvs sphi'fulenses represent the porocytes of Calcarea, 



