44 GENERAL ANATOMY OF THE TISSUES. 



diameter, in which we may distinguish a special investment, the cell mem- 

 brane and contents. The latter are 

 always composed of a fluid, containing 

 formed particles of various kinds, and a 

 peculiar rounded body, the cell-nucleus, 

 which again contains in its interior a 

 fluid and a still smaller corpuscle, the 

 nucleolus. These cells, which must be 

 considered to be endowed with peculiar 

 vital powers, and to be capable of ab- 

 sorption and assimilation, of growth 

 and of multiplication, not only at the earliest period entirely compose 

 the body of the higher and that of most of the lower animals, but almost 

 wholly generate the higher elementary parts of the fully developed body. 

 In fact, even in adult animals, we find in very many places that the ele- 

 ments are simply in the condition of cells, and that as such they take a 

 more or less marked share in the performance of the organic functions. 



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, are unicellular organisms, but 

 it is granted that no demonstration of this has been given in many, 

 especially the Rhizopods. 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,* this is not always clearly demonstrable. 



8. A more exact consideration of the relations of cells give 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. Stel- 

 late (nerve-cells). The size of cells descends upon the one hand, as in 

 many young cells, blood-cells, &c., as low as 0-002-0-003 of a line, 

 and upon the other attains, as in the cysts of the semen and the nerve- 



FiG. 1. Nerve-cells of the Thalamus opticus of man, three of them having their processes 

 torn off. Magnified 3f)0 diam. 



* [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 resem- 

 blance of the gelatinous tissue of the disk of the Medusae to Professor Koiliker's " reticulative 

 connective tissue" is still more striking. TKS.] 



