[Annals N. Y. Acad. Sci., XI, No. i6, pp. 379 to 400, October 13, 1898.] 



THE PHYLOGENETIC SIGNIFICANCE OF CERTAIN 

 PROTOZOAN NUCLEI. 



Gary N. Calkins. 



(Read April ii, 1898.) 



[Plate XXXV.] 



The nucleus is often looked upon as a more or less well-de- 

 fined morphological element of the cell, possessing in its various 

 phases a common type of structure and composed in all cases 

 of similar substances. A comparison of cells in various tissues 

 whether vertebrate or invertebrate, plant or animal, shows that 

 in the majority of cases the nuclei are so similar that, with slight 

 variations, a description of one answers for a description of all. 

 In resting phases the similarity is shown in the distribution of 

 chromatin, linin, and in the nucleoli, while the nuclear membrane 

 is usually present. In active phases metazoan nuclei as a rule, 

 pass through the same stages of spirem-formation, loss of mem- 

 brane, chromosome-formation, and various processes of re-for- 

 mation. The differences between such nuclei being confined 

 mainly to variations in number of chromosomes, in arrangement 

 in the nuclear plate, and in the mode of division. 



The nuclear type being so constant in higher animals we 

 must look to the lower animals — that is, to the Protozoa — to 

 find not only the prototype, but any transitional forms leading 

 up to the highest types, bearing in mind, however, that not- 

 withstanding the constancy of type manifested in the nuclear 

 forms and mitotic processes of the latter, individual differences 

 may have arisen and mitotic processes may have developed in 

 quite diverse ways. In the present paper it is my object to 

 bring together a few facts, some of which are new, showing 

 how in the Protozoa, the nucleus of the type found in Metazoa 



( 379 ) 



