MUSEUM OF COMPARATIVE ZOOLOGY. 263 



cleolar precedes a nucleolar condition, but does not believe, with the latter 

 observer and Klebs ('74), that the nucleoli migrate from the cell protoplasm 

 into the nucleus. Schwalbe agrees with Heitzmann so far as regards the ex- 

 istence of an intra-nuclear network, but not in its connection with an extra- 

 nuclear (or cell) network. 



The principal conclusions reached by R. Hertwig ('76") in his study of nu- 

 clei have been summarized by the author himself as follows : — 



" 1. The most important and characteristic part of the nucleus is the nuclear 

 substance (' Kernsubstanz '), an albuminoid which, though possessing 

 much in common with protoplasm, differs from the latter in numer- 

 ous peculiarities. 

 " 2. The nuclear substance is imbued, to a different extent in different nuclei, 



with a fluid, the nuclear sap (' Kernsaft '). 

 " 3. Primitive nuclei are simply naked masses of this ' nuclear substance.' 

 "4. From this primitive form of nucleus the remaining forms are derived by 

 the following differentiations : — 

 a. The development of a nuclear membrane (nuclei of Infusoria). 

 h. The separation of the nuclear substance from the nuclear sap, 

 whereby the latter 

 a. is irregularly distributed in the nucleus and forms numerous vacu- 

 oles, or 

 /3. is disposed between the nuclear membrane and the nuclear sub- 

 stance, thereby inducing the formation of one or numerous nucleoli. 

 c. The invasion of the nuclear cavity by a nourishing protoplasmic 

 reticulum, which traverses the pores of the membrane and crosses 

 the space occupied by the nuclear sap." 

 " Nuclear substance " is, like protoplasm, capable of automatic motion, which 

 may be either irregular (amoeboid) or executed in a remarkably uniform man- 

 ner, as during cell division ; but in other points — especially in its deportment 

 with acids and with carmine and haematoxylin staining fluids — it shows con- 

 stant differences from cell protoplasm. It is not maintained that " nuclear 

 substance" is everywhere the same, any more than that cell protoplasm is 

 identical in chemical composition in all cells. For the " nuclear sap " specific 

 properties, which would allow it to be recognized as a thing sui generis, have 

 not yet been established. 



Often a portion of the nuclear substance remains in the periphery of the 

 nucleus, thus forming a spherical mantle of homogeneous substance exhibiting 

 the same chemical reactions as the nucleolus. This cortical layer of the nu- 

 cleus (Kernrindenschicht) should not, in Hertwig's opinion, be confounded 

 as has often been the case heretofore, with the nuclear membrane, which is a 

 superimposed structure (Auflagerung), whether derived from the nucleus or 

 from the protoplasm is unknown. The " Kernrindenschicht " is related to the 

 " Kernmembran " much as the cortical layer of the protoplasm of the cell is 

 related to the cell membrane, and like the last-mentioned structure the nuclear 

 membrane is functionally a protective organ, since it cuts off the nucleus in its 



