9 GO SUMMARY OF CURRENT RESEARCHES RELATING TO 



the shells of Ostrca and Mytilus. He arrives at the following con- 

 clusions: — 



Tlie borinf of the canals and galleries is performed solely by the soft 

 })arts of the sponge. The penetration of the prolongation of the body 

 of the sponge into the limy substance of the support appears to be 

 accomplished by the secretion of a corroding liquid, probably an acid. 

 The perf(jration has only been observed in yonng specimens, but it is 

 probable that the same thing takes place in the adult. The secondary 

 canals formed by the prolongations are only the rudiments of the larger 

 canals or galleries; the smooth canals crossing the shell may be for 

 orientation. In species which are very small compared with the mass 

 in which they are buried, the limy substance is simply protective. 

 In other species the sponge encircles the shell as well as penetrates it, 

 and in such cases the support has the same role as any other skeleton, 

 siliceous or calcareous. The destructive action of the sponge is con- 

 siderable. 



Formation of Ova and Spermatozoa in Spongilla fluviatilis.* — 

 Dr. K. Fiedler has made an examination of the developmental history 

 of the generative products of the fresh-water Sponge. A necessary 

 preliminary study is the investigation of the various cell-forms which 

 are found in the mesoderm— or, to use a more indifferent expression — 

 in the layer of connective substance or the internal parenchyma. These 

 cells fall into two groups, for some are cells with regularly, and others 

 with irregularly granular protoplasm. 



The former were long unobserved ; in S. fluviatilis they were first 

 seen by Weltner, and later on, independently, by the author. The 

 graniiles of the protoplasm are spherical, and all of much the same size ; 

 a clear mai'ginal zone of quite transparent protojilasm often remains 

 quite free from granules. The chromatin of the nuclei of these cells 

 has always the form of a more or less fine framework, and as a rule there 

 are no nucleoli in it. There can be no doubt that these cells are capable 

 of amceboid movement. They are scattered through the whole of 

 the body, but are most abundant near the free surfaces. It is very 

 probable that they have a nutritive function. The author enters with 

 some detail into the vexed question of the method of nutrition in Sponges. 



The greater part of the parenchyma of the sponge is made up of cells 

 of the second kind, or those in which the granulation of the protoplasm 

 is irregular. In some of these the nucleus has a filamentar framework 

 of chromatin, in which there are no nucleoli, while others have a rather 

 large, highly refractive nucleolus and very little chromatin ; in the first 

 category are included the slightly specialized ordinary connective-tissue- 

 cells, which, at the time of formation of the generative products, take an 

 important part in the formation of the follicle and Schulze's contractile 

 fibre-cells ; in the second category are the ovarian cells, the silicoblasts, 

 and some forms of amoeboid migratory cells. 



The young ova are distinguished not only by their size and, generally, 

 rounded forms, but by their very clear, because very finely granular, pro- 

 toplasm, and their sharply limited vesicular nucleus, which is large, not 

 only in comparison with other cell-nuclei, but with that of the egg-cell 

 itself. In some cases there is a zone free of granules around the nucleus, 

 and the protoplasm has a rather faint radial striation. The centre of 



* Zeitschr. f. Wiss. Zool., xlvii. (1888) pp. 85 128(2 pis.). 



