The Morphology and Physiology of the Nucleolus. 13 9 
(b) Size of the Nucleolus. — There has been considerable discus- 
sion in the past over the relative sizes of the different cell organs 
at the various stages of development, and we have the opposite 
views of Minot (^f), who attributes senescence to a decrease, and 
Hertwig (85) to an increase, of nuclear as compared with proto- 
plasmic material. Conklin (7), on the other hand, agrees with 
Child that “ senescence seems to be associated with a decrease,, 
and rejuvenescence with an increase of metabolism.” Conklin 
further adds that “ anything which decreases the interchange 
between nucleus and cytoplasm decreases metabolism and leads to 
senescence ; anything which facilitates the interchange increases 
metabolism and leads to rejuvenescence.” 
With regard to the nucleolus, Conklin says : “ The size of the 
nucleoli (plasmosomes) depends upon the size of the nucleus and 
the length of the. resting period ; the larger the nucleus and the 
longer the resting period, the larger the plasmosome becomes. In 
eggs in which the nuclear division has been greatly delayed, if 
not entirely stopped, by the use of hypertonic salt solutions, the 
nucleoli become much larger than in normal eggs.” 
My own observations agree with those of Conklin in some 
respects. That the nucleolus in certain “resting” cells does 
enlarge considerably can be seen by reference to the huge nucleolus 
in the enc^pdermal cell shown in fig. 23. All the figures in my 
third plate from 19 to 28 are drawn to the same scale. The 
nucleoli in the “resting” somatic cells shown in figs. 27 and 28 
are decidedly smaller than those in certain of the embryonic cells, 
e.g. the cells shown in figs. 19, 21, 23 and 26. I am inclined 
therefore to regard the size of the nucleolus as an indication of 
the degree of metabolism existing in the cell — the greater the 
metabolic activity, the larger the total volume of nuclear matter 
present in the nucleus, or extruded in the cytoplasm. In this 
connexion the epithelial cells of fig. 27 should be compared with 
the gland cells of fig. 28. 
(c) Interchange of Materials between Nucleus and Cytoplasm . — 
In considering this aspect of the subject it is important to note : 
(i.) That after each cell division the amount of chromatin com- 
prising the chromosomes in each nucleus is halved; therefore 
during growth and preparation for the next division the amount 
of chromatin has to be increased. We cannot regard the whole of 
the chromosomes as being derived by division from those of the 
egg, and therefore the raw materials which are to be built up to 
form chromatin must be derived from the cytoplasm. Whether 
we can observe the process or not, the nucleus must therefore 
absorb substances from the cytoplasm for both structural and 
metabolic purposes. 
(ii.) That the cytoplasm of the egg possesses potential energy 
in the form of yolk ; there is no such store of reserve food substance 
