18 INTRODUCTION. 
origin in a formless matter (Cytoblastema, germ-substance of cells) ; 
what afterwards remains of this substance may be distinguished as 
Intercellular substance (substantia intercellularis). The cells are 
vesicles, and consist of a fine membrane which encloses a fluid often 
containing granules. For the most part these cells have a so-called 
nucleus, a small dark-coloured corpuscle, lying on the wall of the 
cell. In this nucleus a round spot has been distinguished and 
termed nucleolus. The formation of these cells seems to proceed not 
always in the same manner. According to SCHWANN a nucleolus 
arises first, round this a nucleus is formed as its envelope, by the 
ageregation of granules in the fluid germ-substance: at a slight 
distance from this nucleus there coagulates, as it were, a thin mem- 
brane, the Cell-wall, which at first is raised, like a watch-glass, on 
one side of the nucleus, and afterwards encloses it all round. On 
this account the nucleus is considered to be the germ of the cell 
(Cytoblastus); when the cell is formed, the nucleus, according to 
ScHWANN, has discharged its office: it is detached and disappears. 
The researches of HENLE? have shewn that such is not universally 
the case, but that in fibrous tissues formed from cells, the cell- 
nucleus is changed into peculiar fibres. 
Cells when once formed are multiplied by fission, or by the 
formation of new cells within those already formed. The parts 
then of those tissues, with which we have become acquainted 
above, are either cells or fibres which have been formed from cells. 
(1) In some tissues the cells, which have been plainly isolated, are 
present as elements at a later period, as in adipose tissue and 
cuticle; (2) in other tissues the walls of the cells become thick- 
ened, and coalesce with one another and with the intercellular sub- 
stance, whilst the cavities remain separate, as in cartilage; (3) in 
others, again, the cavities coalesce, whilst the walls of the cells 
that mutually touch, are destroyed or absorbed. Finally, other 
tissues, still, exhibit as elementary parts little plates without 
cavities, which may probably have existed at an earlier period. 
These either join one another in a plane, or range themselves 
lengthwise in a row, as in the fibres of organic muscles and of 
Conjunctive Tissue. Other fibres may, according to HENLE, be con- 
sidered as compound cells, 7.e. those whose nucleus was originally 
1 HENLE, Allgemeine Anatomie, s. 188—9. 
