ZOOLOGY AND BOTANY, MICROSCOPY ETC. 421 



excentric layers found, but a complexity is introduced as soon as the sap- 

 vacuoles appear, in many cases making the cell not monocentric but poly- 

 centric. The normal order of the layers, as exemplified by the spore of 

 Equisetum, or any simple cell with one large vacuole, &c., may be distin- 

 guished from the inverse order exhibited, for instance, by the cords in a 

 Caulerpa, or the central mass in a cell containing raphides, or anywhere 

 where the sap bathes the system of layers referred to. 



It is then shown that in many cases where oil-drops, &c., have usually 

 been regarded as lying free in a cell, they are inclosed in an ingrowth from 

 the cell-wall, reminding us of cystoliths. An examination of intercellular 

 spaces follows : the most interesting question is as to the existence of pro- 

 toplasm in lacun8B between cells. Berthold goes much further in this 

 respect than other writers. He finds a thin layer of protoplasm overlying 

 the cuticle of the epidermis and of spores, and concludes that the cell-wall 

 is formed and imbedded in protoplasm, and not excreted on its surface — 

 the cell-wall is a supporting apparatus, not a protective one. Again, a 

 cell forming part of a tissue cannot be forthwith compared with a unicellu- 

 lar alga, for this reason : the latter may be regarded as consisting of two 

 parts, (1) the inner protoplasmic system with its contiguous share of cell- 

 wall, (2) the outer strata of cell-wall plus the hypothetical covering of 

 protoplasm. Only the first of these two parts of the algal cell can be com- 

 pared with a tissue-cell. 



The second chapter is concerned with the finer structure of the cell- 

 nucleus, chlorophyll-corpuscles, and other cell-contents. It is stated that 

 starch is not formed in the MelanophyceaB, and that the word " microsome " 

 has no definite meaning, and had better be discarded. 



If protoplasm is an emulsion, it follows that the various processes of 

 separation of sap-vacuoles, oil-drops, crystalline and other particles, have to 

 be explained as according with similar separations in lifeless mixtures ; and 

 this the author maintains to be the case. 



The supposition that anything is explained by regarding protoplasm as 

 essentially " living proteid," is severely criticized, and the author agrees 

 with Baumann that the arguments which exalt proteids into the position of 

 being the most essential constituent of protoplasm would apply equally well 

 to water. The " living substance of organisms " is always an extremely 

 complex mixture. Berthold proposes to recast the definition of protoplasm, 

 and to subordinate to it — the fluid mixture absent from no living cell — 

 cytoplasm, nucleus, chlorophyll-bodies, vacuoles, tannin and oil-drops, &c., 

 as so many parts of the protoplasm as a whole. 



In the third chapter is an analysis of the movements of naked masses 

 of protoplasm. All turns upon the tendency of a mass of protoplasm to 

 assume the form of a spherical drop ; this can only be due to the same 

 causes which impel a drop of any accepted liquid to assume the drop con- 

 dition. The amoeboid condition depends upon the degree of wetting of the 

 environment by the fluid protoplasm, and vice versa. If three fluids which 

 do not mix are in contact with one another, the tensions at their surfaces 

 can be mathematically investigated, and Berthold maintains that the 

 principles here concerned govern the behaviour of a drop of protoplasm 

 as they do that of an ordinary liquid under the given conditions. The 

 phenomena of spreading out, putting forth and withdrawing pseudopodia, 

 rounding ofi", &c., are due to the same causes and ruled by the same laws as 

 the flowing of one liquid over another, or its withdrawal from it (glycerin 

 and alcohol, for example), or its assumption of the drop form, and so on. 



The fourth chapter deals with the symmetry or arrangement of the cell- 

 contents. The stratified or shell arrangement is again expressly referred to, 



