ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 175 



true enamel fibres probably separate from the enamel membrane, coa- 

 lesce with the enamel already formed, and at the same time become im- 

 pregnated with calcareous salts. The substantia propria or ivory of the 

 tooth is formed of fibres, between which the dental tubuli run. The 

 pulp of the tooth at its surface consists of cylindrical cells which con- 

 tain nuclei with nucleoli. The interior of the pulp is composed of 

 round nucleated cells. Schwann conjectures that the fibres at the sur- 

 face of the pulp are in successive layers added to and converted into 

 the growing dental substance. 



CLASS IV. 1. Cellular Tissue. In the development of cellular tissue 

 there first appears a structureless cytoblastema, in which round nuclea- 

 ted cells are subsequently formed. These cells become transformed into 

 spindle-shaped bodies, which have in their interior, but attached to their 

 wall, a round or oval nucleus, while this nucleus in its turn includes 

 one or two dark points (nucleoli). These elongated cells become more 

 and more drawn out at their extremities, and give off fibres, which are 

 sometimes branched ; and at length become resolved at each end into a 

 fasciculus of extremely delicate fibrils. The division of the fibre-like 

 prolongations of the cells into more minute fibrils gradually extends to- 

 wards the centre of the cell, so that at a later period the fasciculus 

 of fibrils proceeds immediately from the body of the cell. Lastly, 

 the division into fibrils takes place even in the situation of the nu- 

 cleus of the cell, and then the cell becomes wholly resolved into 

 a fasciculus of fibres, upon which the nucleus lies. The fibres are 

 probably tubular. 



The cells of adipose tissue which are found even in the cellular tissue 

 of the foetus, present at first a distinct nucleus attached to their mem- 

 branous wall. When the wall of the cell is thin, the nucleus forms 

 a prominence above the surface of the fat globule contained in the 

 cell. When the wall of the cell is thick, the nucleus is entirely 

 included in its thickness. The nucleus contains one or two nu- 

 cleoli. The fat cells in the cranium of a young fish (Plotze,) some- 

 times have each two nuclei which bear the same relation to the mem- 

 branous wall of the cell. In the cellular tissue of the foetus a third 

 kind of cells is met with. These are round and pale ; each has a nu- 

 cleus with one or two nucleoli attached to their wall ; they do not be- 

 come elongated into fibres, contain no fat, but are filled with granules ; 

 and this deposit of granules is first formed about the nucleus. 



The cellular tissue of the foetus, when submitted to boiling, yields 

 no gelatine ; but in its place a substance which resembles pyine, except 



