CHAPTER VIII 



SPECIAL MORPHOLOGY OF THE FROTOZOAN NUCLEUS 



" By cell-division, accordingly, the hereditary substance is split off from the parent-body; 

 and by cell-division, again, this substance is handed on by the fertilized egg-cell or oosperm 

 to every part of the body arising from it. Cell-division is, therefore, one of the central 

 facts of development and inheritance." — Wilson. 1 



It has been shown in the foregoing chapters that local modifica- 

 tions of the general protoplasmic basis of protozoan cells give rise to 

 cell-structures of considerable complexity, and, when compared with 

 cells in the tissues of Metazoa, these complex forms often appear to 

 be the more highly differentiated. These differentiations have 

 mainly arisen by modifications of the cortical plasm, in response, prob- 

 ably, to the action of the environment and the mode of life. The 

 differentiations of the inner cell-plasm of the Protozoa, on the other 

 hand, are apparently much less complex than in Metazoa, and the 

 nucleus, especially, appears to be structurally much more simple in 

 the Protozoa. In the division of protozoan cells, structures occur 

 which are undoubtedly similar to those of tissue-cells, but are, on the 

 whole, of a simpler and more generalized character. For this reason 

 the careful study of these more primitive organs cannot fail to throw 

 light upon morphological problems which, in Metazoa and the higher 

 plants, are more difficult of solution. 



The changes which the nuclei undergo during division of the cell 

 have received little attention from the phylogenetic standpoint, 

 although the similarity of these changes, even in the most diverse 

 tissues of unrelated animals and plants, bespeaks a common origin. 

 If the structures involved in these changes can be traced back to 

 more generalized organs in the Protozoa, a key may perhaps be found, 

 — not, indeed, to ancestry of the Metazoa, as some writers have main- 

 tained, — but to some of the problems of higher cell-differentiations. 

 It is my purpose in the present chapter, therefore, to give somewhat 

 in detail a comparison of the various structures found in protozoan 

 cells with those of the cells in differentiated tissues of Metazoa and 

 Metaphyta, and so far as our present knowledge permits, to point out 

 the possible origin of different cell-organs in higher types, from the 

 more primitive structures as they appear in Protozoa. 



i The Cell, p. 63. 

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