28 



GENEKAL PHYSIOLOGY 



retreat into the deeper layers. This is also true of the nuclei of many 

 secretory glands, such as the silk glands of different lepidoptera. 



The nucleus, therefore, must be regarded not only as a necessary 

 constituent of the cell, but as its most important constructive ele- 

 ment. To be sure, many cells, such as the erythrocytes of the mam- 

 malian blood, are capable of leading an independent existence even 

 without a nucleus, but this example can scarcely be used as a proof 

 against the preceding statement, because these cells are nucleated 

 when formed and do not possess the power of regeneration. Even the 

 bacteria form only an apparent exception, because their nuclear 

 material is either widely disseminated through the cell in the form of 

 dust-like granules or is already arranged as spores. Obviously, 



FIG. 4. REGENEBATION OF STENTOR ROESELII. 



A. Stentor divided into two nucleated portions; B and C newly formed organisms. 

 (Verworn.) 



therefore, the chemical and structural development of the cell depends 

 upon the nucleus. To some extent, however, it is also true that a 

 nucleus devoid of cytoplasm, cannot exist as an independent entity. 

 To be sure, in many cells the protoplasmic envelope is extremely 

 narrow and in many it does not seem to be present at all. But, the 

 spermatozoa, to which reference is now had, are not capable of trans- 

 formation nor of multiplication, their sole purpose being to unite 

 with the ova. Verworn, moreover, has shown that the isolated nucleus 

 of the large radiolaria does not long survive its removal from the cell. 

 These functional differences between the cytoplasm and the nucleus 

 are associated with definite chemical differences. This may be 

 inferred from the important changes which the nucleus undergoes 



