CELLS. 1 5 



[It is not yet quite decided what part cells play in the 

 composition of the simplest animals. Siebold and I have 

 expressed the opinion that the Protozoa, like the simplest 

 plants, arc unicellular organisms, but it is granted that no 

 demonstration of this has been given in many, especially the 

 Kliizopods. In all creatures above the Protozoa, it would seem 

 to be certain that their body proceeds from a mass of cells, 

 although in the fully-formed animals, as for example the 

 Hydra, according to Ecker, 1 this is not always clearly de- 

 monstrable.] 



§ 8. 



A more exact consideration of the relations of cells gives us 

 the following results. Their fundamental form is globular or 

 lenticular ; it is such in all cells during their earliest state, 

 and is permanent in those which occur in the fluids, (blood- 

 corpuscles, &c.). Less common forms are: 1. Polygonal 

 (pavement epithelium). 2. Conical or pyramidal (ciliated 

 epithelium). 3. Cylindrical (cylinder epithelium). 4. Spindle- 

 shaped (contractile fibre cells). 5. Squamous (epidermic scales). 

 6. Stellate (nerve-cells). The size of cells descends upon the 

 one hand, as in many young cells, blood-cells, &c, as low as 

 0002 — 0008"', and upon the other attains, as in the cysts of 

 the semen and the nerve-cells, that of 002 — 0-04'". The largest 

 animal cells are the yelk-cells or ova, especially those of birds 

 and amphibia, and a few of those animals which consist of 

 single cells, these, in certain Gregarinse, attaining 0"7'". 



The membrane of the cells is generally very delicate, smooth, 

 hardly separable, and marked by a single contour, rarely of any 

 considerable density or measurable thickness ; with our present 

 optical instruments it exhibits no structure of any kind. In 

 the interior of the cells there are invariably found, at a certain 

 time, one or many nuclei, besides fluid and granules of various 

 proportions and of different natures. Cells which contain only 



[' The tissue of the Hydra presents no essential features of difference from that 

 of the higher animals, and closely resembles the most superficial layer of the dermis 

 in the latter. In its outer part the so-called nuclei are almost wholly converted 

 into thread-cells; but they may be very readily demonstrated in their ordinary 

 condition in the deeper portions. The resemblance of the gelatinous tissue of the 

 disk of the Medusa? to Professor Kolliker's " reticulated connective-tissue" is still 

 more striking. — Eds.] 



