INTRODUCTION 



pointed "pseudopodia" from their superficial protoplasm (Zygoco- 

 metes and others). It seems that in any attempt at a phylogeny of 

 the Protozoa we should have to treat the assemblage of forms now 

 classed as Mastigophora (Flagellata) as a central group from which 

 the other eight classes have been derived, whilst embracing in 

 itself several specialised lines of descent, including that which has 

 given rise to the primitive green plants. 



The indication of a higher and later elaboration of structure, 

 as distinct from a lower and more primitive, by means of the 

 classifieatory artifice of " grades," has, however, been introduced in 

 the present work by Professor Hickson in regard to the classes of 

 Protozoa by a consideration of the cell -nucleus. The condition 

 of this important structure justifies, he considers, the separation of 

 the classes of Protozoa into a lower and a higher grade the 

 Homokaryota and the Heterokaryota and it is not improbable 

 that further study of the lower grade will lead to the subdivision 

 of that assemblage into sub-grades. 



The history of the nucleus of the corpuscle of protoplasm, that 

 corpuscle which it is customary to regard under the name of " the 

 cell " as the unit of living structure, is at present absolutely un- 

 known and altogether a matter of conjecture. It may perhaps be 

 conceded as highly probable that the earliest protoplasm was with- 

 out nucleus or differentiated nuclear material. It is a legitimate 

 contention that such a substance should not be called " protoplasm " 

 at all, since Hugo von Mohl- invented this term to describe the 

 viscid contents of a vegetable cell expressly including the nucleus 

 a? part of it. It was proposed some twenty-five years ago by 

 Ed. van Beneden to call the earlier non-nucleated stage of living 

 matter "plasson," and it seems to me by adopting this term we 

 can preserve the word " protoplasm " for its original use. At the 

 same time it is important to avoid using the word " protoplasm," as 

 is not unfrequently done, to signify the critical chemical body which 

 undoubtedly is present in living protoplasm and is the apex of the 

 pyramid or the top of the fountain, to which a variety of chemical 

 bodies are leading and from which another series of chemical bodies 

 are receding at every moment of the chemical activity of living 

 protoplasm. Protoplasm is not a chemical body but a structure, 

 and its nuclear particles, as well as its definitely formed nucleus 

 consisting of chromatin and other constituents, are parts of it. It 

 seems necessary to have a word by which to refer to the highest 

 group of chemical molecules to which one set of chemical processes 

 in the cell are always leading and from which another series are reced- 

 ing. I proposed some years ago (Ency. Brit., article "Zoology") 

 to speak of this hypothetical body as " plasmogen." In the same 

 way it is necessary to avoid the tendency which exists to employ 

 the word " protoplasm " to describe cell-substance both when con- 



