CHROMOSOMES AND CYTOPLASM IN PROTOZOA 



47 



these matters and to extend our knowledge 

 in these directions. 



To present these things effectively I must 

 recall important features in the relations 

 of nucleus and cytoplasm in these organ- 

 isms, as well as certain striking features in 

 their biology. The organisms with which 

 we are concerned are the ciliate infusoria, 

 familiar to all of us in the school example 

 Paramecnim, or in other species. 



These organisms begin their life cycle 

 immediately after conjugation with a single 

 diploid nucleus ; half of its chromosomes are 

 derived from one of the two parents that 

 have united in conjugation and half from 

 the other. This single nucleus is in the 

 midst of a complex body, the varied sub- 

 stances of which are known collectively as 

 the cytoplasm. This cytoplasmic body is 

 a highly organized structure, with protec- 

 tive parts, contractile fibers, transmissory 

 fibers that tempt one to speak of a nervous 

 system, motile organs (or organelles if you 

 prefer), a partially differentiated alimen- 

 tary canal (often with a complex armored 

 pharynx), and other structures, as definite 

 and as functionally arranged and opera- 

 tive as the organ systems of some of the 

 multicellular creatures. To think of the 

 cytoplasm in these organisms as an undif- 

 ferentiated mass of material is to totally 

 misconceive the situation ; it has an extraor- 

 dinarily complex organization. 



In the infusorian there is interaction be- 

 tween the nucleus and the cytoplasmic parts 

 comparable to what I have sketched in the 

 ovum of multicellular organisms. But in 

 the Ciliate, this interaction is highly devel- 

 oped, involving differentiated parts not seen 

 in most cells. 



The single diploid nucleus that is present 

 after conjugation soon gives off a portion 

 of itself which is destined for interaction 

 with the cytoplasm. The details differ in 

 different species, but the essential feature 

 is that the nucleus divides into two parts. 

 One of these absorbs much material from 

 the cytoplasm, becomes greatly enlarged, 

 loses its sharply defined chromosomal struc- 

 ture, and eventually becomes completely 

 absorbed into the cytoplasm. This part. 



commonly known as the macronueleus, cor- 

 responds in function to the chromatin gran- 

 ules and so-called "nuclear sap" that in 

 the ovum of multicellular organisms are 

 given off to the cytoplasm by the nucleus. 

 But in the infusorian this mass of nuclear 

 material for a long time persists as a definite 

 body. It is only after a number of vegeta- 

 tive generations — few or many — that it is 

 absorbed into the cytoplasm. In many 

 species this occurs just before or during 

 conjugation ; in others, such as Paramecium 

 aurelia and Paramecium caudatum, it 

 . occurs during vegetative life at fairly regu- 

 lar intervals of 20 to 60 generations. The 

 dissolution of this great mass of nuclear 

 material into the cytoplasm is a particularly 

 striking example of the fact that nucleus 

 and cytoplasm are not wholly separate, but 

 that material continually transforms from 

 one to the other. The transfer of so great 

 a quantity of nuclear material into the cyto- 

 plasm must greatly affect the nature and 

 physiological activity of the cytoplasm; 

 some of its presumable results we shall see 

 later in the genetics of these organisms. 



The production of the highly differen- 

 tiated organization of the cytoplasmic body 

 in these animals presents a problem for 

 experimental embryology that appears 

 more difficult even than does differentiation 

 in multicellular organisms. The schema 

 which I outlined for the interaction of 

 nucleus and cytoplasm in producing the 

 bodily differentiations in multicellular or- 

 ganisms appears to break down in its 

 application to the development of complex 

 Protozoa. That schema included differen- 

 tiations of the cytoplasm produced by inter- 

 action with the nucleus; the segregation of 

 the diverse cytoplasmic materials in differ- 

 ent cells, each with its nucleus ; the further 

 interaction of the diverse cytoplasms of the 

 different cells each with its own nucleus, 

 yielding diverse products in the different 

 cells; and further segregation by cell divi- 

 sion. But here in the infusorian the multi- 

 farious differentiations all occur in a single 

 minute mass of cytoplasm in the presence 

 of but a single nucleus. When experimen- 

 tal embryology has satisfied itself on the 



