CHROMOSOMES AND CYTOPLASM IN PROTOZOA 



By H. S. JENNINGS 



DKPAETMENT OF ZOOLOGY, THE UNIVERSITY OF CALIFORNIA AT LOS ANGELES, LOS ANGELES, CALIF. 



My subject is the interaction of chromo- 

 somes and cytoplasm in genetics and in 

 development, with special emphasis on the 

 role of cytoplasm. I shall first treat this 

 subject generally. Then I hope to illus- 

 trate, and perhaps to illuminate, some of 

 these relations — particularly those con- 

 cerned with genetics — from experimental 

 observations on some of the more complex 

 Protozoa. 



Many years ago a well-known investiga- 

 tor whose devotion was all to biological 

 chemistry ridiculed the notion, then coming 

 into prominence, that chromosomes have an 

 important role in inheritance and develop- 

 ment, characterizing them in their supposed 

 fixity and inertness as ' ' bullets. ' ' Possibly 

 some recent theories of the unitary nature 

 of genes might, for the genes, offer tempta- 

 tion to such a gibe. 



But chromosomes as perceptual objects 

 in space show none of the fixity and inert- 

 ness of bullets. The pictures that we see 

 of the condensed stages of chromosomes, at 

 the time when they are undergoing division, 

 are most misleading if they are assumed to 

 show the distinctive characteristics of chro- 

 mosomes. The chromosomes are active and 

 changeable ; they continually operate on the 

 remainder of the cell (which we lump to- 

 gether as cytoplasm), visibly interchanging 

 materials with the cytoplasm, altering it, 

 differentiating it. Recall a few of the fun- 

 damental features of such interaction of 

 chromosomes and cytoplasm. 



In the egg of Fundulus, according to the 

 work of Richards (1917), the changes in the 

 chromosomes, though visible in normal de- 

 velopment, are rendered more conspicuous 

 by brief exposure to radiation. Then the 

 following changes are seen clearly. The 

 condensed chromosomes begin to enlarge 

 and to take in material from the cytoplasm, 

 so becoming vesicular. The vesicles in- 



crease in size; they take in so much cyto- 

 plasmic material that each chromosome 

 forms a vesicle hundreds of times the vol- 

 ume of the same chromosome in the con- 

 densed condition. The chromosomal vesi- 

 cles become so large that their boundaries 

 touch and they are pressed together. The 

 whole group of vesicles now constitutes the 

 nucleus. 



The chromosomes then are not merely the 

 minute dense structures ordinarily called 

 chromosomes. For the greater part of their 

 lives they are large vesicles enclosing much 

 cytoplasmic material. Such chromosomal 

 vesicles may be seen and counted in the 

 so-called resting nucleus of many cells; 

 they are described under the name of pro- 

 chromosomes. 



These vesicles are active; they are con- 

 tinually changing. At their maximum size 

 they contain a large quantity of material 

 absorbed from the cytoplasm. What are 

 they doing with this material? 



"What they are doing with it is learned 

 partly from cytological observations and 

 partly from our knowledge of the activities 

 of chromosomes, acquired in the study of 

 genetics. Direct observation shows that 

 after the vesicles have become very large 

 by absorbing much cytoplasmic material, 

 they discharge that material back into the 

 cytoplasm. The membranes of the vesicles 

 fade away. The contained materials pass 

 out into the cell body, where they are again 

 classified as cytoplasm. The condensed 

 chromosomes for the next cell division are 

 formed from minute reserve parts of the 

 large vesicles. 



Direct observation shows further that the 

 materials thus returned to the cytoplasm 

 have been greatly altered while in the 

 chromosomal vesicles. They carry with 

 them great numbers of chromatin particles, 

 detectable by cytological methods. 



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