THE CELL. 33 



to consist of a very delicate network or reticulum of minute fibres, 

 termed the spongioplasm. The points of junction of these fibres 

 and their optical cross-sections give a finely granular appearance to 

 the cytoplasm. 



In the meshes of the spongioplasm is a clear, homogeneous sub- 

 stance, the hyaloplasm. This may also contain some granules, but 

 they are probably not constituent parts of the cytoplasm and are 

 grouped under the term metaplasm. Some of them are composed 

 of material taken from without, either in their original form or 

 slightly modified ; others have been produced within the cell by 

 chemical transformations, and are either useful products, to be sub- 

 sequently turned to account by the cell itself or to be discharged as 

 a secretion, or they are waste matter destined for elimination from 

 the body. 



The relative proportions of the hyaloplasm and the spongioplasm 

 and the arrangement of the fibres of the latter both vary in differ- 

 ent cells. 1 



When seen under the microscope the structure of the nucleus, 

 except during the division of the cell, closely resembles that of the 

 cytoplasm. It is traversed by a number of delicate fibres, which 

 branch and give the nucleus a reticulated appearance. At its sur- 

 face these filaments unite to form a delicate membranous envelope, 

 sharply defining the nucleus from the surrounding cytoplasm, but 

 it is a question whether this membrane is continuous, or whether it 

 is an exceedingly close meshwork with minute apertures permitting 

 a direct communication between the cytoplasm and the interior of 

 the nucleus. 



The spaces between the nuclear filaments are occupied by a clear, 

 homogeneous substance, which may be identical and continuous with 

 the hyaloplasm of the rest of the cell. 



One or more highly refracting bodies, the nucleoli, may be pres- 

 ent in the nucleus, lying freely in the clear substance between the 

 filaments or attached to the latter. Their purpose is not known, 

 but it is thought that they are not essential parts of the cell but 

 correspond more or less closely to the metaplasm in the cell-body. 



1 The reticulated appearance of the cytoplasm may also be explained by assum- 

 ing it to have an alveolar structure, and the theory that such is its actual structure 

 possesses much plausibility. In that case the visible reticulum would be formed by 

 the walls of the alveoli and their lines and points of intersection, all of which 

 would be included in the spongioplasm, while the contents of the alveoli would 

 constitute the hyaloplasm. 

 3 



