54 



stained with Heidenhain's iron-haematoxylin the nucleoli appear black ; in addition we see 

 numerous black granules in the centre; whether these belong to the nucleus or are situated 

 around it I am not sure of; I believe the latter to be the case since I found some granules 

 beyond the nucleus, more towards the periphery (PI. VI, fig. 12). What are these cells? Their 

 position in most specimens, accompanying the bundies of tylostyles might suggest that they 

 are scleroblasts. I failed however to find one case in which a young tylostyle was to be seen 

 inside it. On the other hand it struck me that between the cells packed together in fusiform 

 strings numerous fibres vvere always visible. Often longitudinal stripes were visible between one 

 nucleus and the following, resembling on the whole the pictures Flemming *) gives of the 

 developement of fibres in the peripheral parts of cells. I believe indeed that the cells under 

 consideration are the mother cells of the connective tissue fibres which are numerous between 

 and arround the tylostyles in a bundie. I feel the more inclined to believe that these fusiform 

 cells with their large ellipsoidic nuclei are fibroblasts, as we may call them, since everywhere 

 at the periphery of the sponge, where similar cells occur, situated with their long axes parallel 

 to the surface, the same abundance of fibres occurs. 



As stated above, true muscle cells occur, especially in the tissue surrounding the large 

 central canals. These cells are at once distinguished from other cells by their elongated, rod- 

 like nucleus (PI. VII, figs. 2, 4 and 6). The cells are thread-like (fig. 2) and generally occur 

 in bundies as is shown by figs. 4 and 6. The majority of these cells are situated in concentric 

 layers near the lumen of the large central canals. But some other bundies occur perpendicular 

 to the former (fig. 4). If the concentric muscle cells are dilated, the radial ones contracted, 

 the tissue lining the canal will be contracted and the rugae mentioned above will be distinctly 

 seen (PI. V, figs. 4 and 11). Longitudinal sections show the wall of the canal festoned 

 (PI. V, figs. 12 and 18; PI. VI, fig. 5). If on the contrary the concentric muscle cells 

 contract, the rugae will form membraneous diaphragma's ; the more they are contracted the 

 narrower the canal will be. This we see on a large scale in the large central canals, but also 

 in other canals. 



Little is known about the remarkable, purplish color some specimens exhibit. Mac Munn 

 (1900) showed that "this pigment is characterised by possessing a very well-marked banded 

 absorption spectrum, and by certain other characteristics which distinguish it from other pigments 

 which have been hitherto described". The most remarkable feature is that it is not soluble in 

 alcohol, benzol, ether, chloroform, ammonia; nor is it destroyed by exposure to air. Consequently 

 we see specimens preserved in alcohol or dried as brightly colored as the living specimen. 

 Topsent remarked (1906^ p. 573): "la substance colorante impregne tous les tissus, mais une 

 coupe montée au baume met en évidence, nombreuse dans 1'écorce, des cellules plus intensément 

 colorées que tout ce qui les entoure". As far as I can see only the first part of this sentence 

 is true ; a microscopical section through B. M. 33 shows all the various cells equally stained, 

 the nuclei as well as the cytoplasma, nucleoli etc. The nuclei and nucleoli being thicker and 

 more compact naturally look somewhat darker. And so do places, where cells are situated 



1) Flemming in Hertwig's Handb. Entw. I.ehre. III, p. 6. 



