20 INTRODUCTION. 



litmus may change to an acid reaction due to the formation of paralactic 

 acid. Also on the death of the cell other new bodies may be produced, 

 or cell constituents, like glycogen, may be consumed and also constituents 

 may pass from the cell into the surrounding fluid and hence be lost for 

 the investigation. 



There are still other difficulties that must not be underestimated. 

 The constituents occurring in the cells are not all of the same physio- 

 logical importance. Some of these are essentially necessary for the 

 life of the cells, while others are only considered as stored -up reserve 

 material or as metabolic products. In this connection we have only 

 been able, thus far, to learn of certain substances which seem to occur 

 in every developing cell. Such bodies, called PRIMARY by KOSSEL/ 

 are, besides water and certain mineral constituents, proteins, nucleo- 

 proteins or nucleoalbumins, phosphatides (lecithin), glycogen (?), 

 and cholesterin. Those bodies which do not occur in every developing 

 cell are called SECONDARY. Among these we have fat, glycogen (?), 

 pigments, etc. It must not be forgotten that it is still possible that 

 other primary cell constituents may exist, as yet unknown to us, and 

 we also do not know whether all the primary constituents of the cell 

 are necessary or essential for its life and functions. 



Another important question is the division of the various cell con- 

 stituents between the two morphological components of the cell, namely, 

 the protoplasm and the nucleus. This is very difficult to decide, for 

 many of the constituents and even the division of the chief chemical 

 constituent, the proteins, between the protoplasm and nucleus, have been 

 but little studied. It is very probable that the nucleus (in abundance) 

 as well as the protoplasm contains the conjugated proteins, which have 

 been called nucleoproteins, and which will be discussed in detail in a 

 following chapter (III) . The nucleoproteins of the protoplasm are richer 

 in the protein component and poorer iri the phosphorized component 

 as compared with the nucleoproteins of the nucleus, which also have a 

 more marked acid character. 



The question as to the occurrence of a special external boundary 

 layer in the cells is of importance for the understanding of the metab- 

 olism of the cells, as well as for our knowledge as to the manner in which 

 the intake and output of bodies transpires in the cells. In this connec- 

 tion we must call attention to the fact that in certain cells an external 

 thick layer or a true membrane exists which seems to consist of protein 

 substance. Even cells in which no special external boundary layer 

 can be observed are still considered as having such a layer because of the 

 permeability conditions. 



1 Verhandl. d. physiol. Gesellsch. zu Berlin, 1890-91, Nos. 5 and 6. 



