446 M. J. Brock on the Occurrence of 



and not unfrequently, especially in alcoliolic preparations, 

 strikingly stelliform, to which Prof. Graf Solras called my 

 attention. The increase by transverse division, which is 

 frequently to be observed, may also be cited among the cri- 

 teria of cell-nature. Further, they are spherical bodies of 6- 

 8 /A in diameter (nucleus 2 /a), the contour of which appears 

 so sharply and definitely that the assumption of a special 

 (cellulose?) envelope* seems to be justified. From the 

 numerous vacuoles which permeate it the protoplasm has a 

 frothy character ; usually a ring of larger vacuoles surrounds 

 the nucleus, and between this and the membrane there are 

 numerous smaller ones. But the most multifarious other 

 arrangements also occur. The green colouring-matter, which 

 is fixed by chromic acid but extracted by alcohol^ is not gene- 

 rally diffused through the protoplasm, but localized in small 

 round corpuscles (chlorophyll-bearers), which are distributed 

 through the cell in variable numbers difficult to determine. 

 However, their quantity suffices to cause the whole cell to 

 appear of a lively green colour under low powers. Whether 

 the green granules are situated in the vacuoles or in the pro- 

 toplasm is difficult to decide from sections ; but I regard the 

 latter as far more probable. 



Other points in the structure of the pseudochlorophyll-cor- 

 puscles I have been unable to make out clearly. In spirit- 

 preparations, in which, as already stated, the green colouring- 

 matter has been entirely extracted, the granules of the proto- 

 plasm, which bore the colouring-matter, have also become 

 very indistinct, as their refractive power too nearly approaches 

 that of the rest of the protoplasm. It is only where (in the 

 microscopic image) they lie over a large vacuole that they are 

 very distinctly visible. The vacuoles, however, in their form 

 and distribution are, on the contrary, particularly clearly seen 

 in spirit-preparations from which the colouring-matter has 

 disappeared. The very tine, strongly refractive, almost dust- 

 like granules which I sometimes found scattered through the 

 protoplasm have remained quite inexplicable to me as regards 

 their nature and significance ; but I must mention that on 

 treating sections of Tridacna hardened in osmium with 

 iodized solution of iodide of potassium for a very different 

 purpose (see p. 450) , fine, dust-like, violet-blue granules made 

 their appearance in many of the green cells, while in spirit- 

 preparations chloride of zinc and iodine coloured the whole of 

 the cell-contents deep blue-black. How these two results 

 are to be reconciled, and whether the blue granules are ideu- 



* The test with chloride of ziuc aud iodine was not unequivocally 

 successful. 



