450 M. J. Brock on the 



cells, the perfectly hyaline protoplasm of which is throughout 

 so completely stuffed with strongly refractive granules of a 

 fatty lustre that I could not even find a cell-nucleus. The 

 granules of the contents, averaging O'5-l /i in diameter, are 

 irregularly polygonal rather than rounded : in osmium they 

 become brown rather more strongly than the protoplasm, and 

 also acquire a deep colour in borax-carmine. 



We find these " granule-cells," which usually attain twice 

 or three times the size of the ordinary blood-cells, intermixed 

 with these and the Algge in the blood-lacunse, where they 

 generally lie close to the walls, often in recess-like depres- 

 sions. If such a depression is seen from the side in a section 

 peculiar images are produced, as though the cell lay free in 

 the interlacunar tissue ; and misled by this, I thought for a 

 long time that I had to do with true wandering cells, but I 

 gave up this notion on finding that the explanation above 

 given is quite sufficient. 



The frequency of the " granule-cells " is very variable. 

 They are wanting in none of my three individuals of Tri- 

 dacna ; but while in the specimen treated with chromic acid 

 and alcohol they always occur singly and so sparingly that I 

 often had to examine several sections in order to find one, 

 they occurred in the osmium-specimen in such abundance 

 that every section showed at least half a dozen of them. The 

 causes of this phenomenon are quite unknown to me. 



In order to understand these peculiar cells it is most neces- 

 sary to make out the chemical nature of their contained cor- 

 puscles. For reasons which are not far to seek I at first 

 thought of glycogen ; but I did not succeed in obtaining the 

 characteristic glycogen-reaction with a solution of iodine and 

 iodide of potassium prepared in accordance with Barfurth's 

 prescription*. Whether this failure is to be ascribed to the 

 hardening with osmium or to the processes of imbedding in 

 paraffin may be decided by better chemists than myself — at 

 any rate from the rarity of the " granule-cells " in my other 

 two Tridacnce I was limited to the osmium-specimen for this 

 microchemical test. 



The resemblance of the " granule-cells " to certain cells of 

 the interstitial connective substance of the Pulmonata, which 

 were discovered by Semper and reinvestigated and further 

 described by me some years ago f, is very remarkable. The 



* D. Barfurtli, " Vergleichend-histochemisclie Untersucliungeu iiber 

 das Glycogen," in Arcli. fiirmikr. Anat. Bd. xxv. p. 260. 



t J. Brock, " Untersuchungen iiber die interstitiellen Bindesubstauzen 

 der Mollusken," in Zeitscbr. f. wiss. Zool. Bd. xxxix. p. 40 (1883), 



