U L T R A S T R U C T U R E OF T H E A M P H I B I A N EGG
195
Nucleoli are free floating bodies found in the nuclear sap. By means
of the phase-contrast microscope they can be observed to be composed
of several different types. Some correspond to those found on both the
axis and the loops of the chromosomes. For a long time, it was assumed
that the peripheral nucleoli, which are the most conspicuous of all
free-floating bodies, originated from specific sites on the chromosome
FIG. 25. A three-dimensional diagrammatic representation of the amphibian
oocyte nucleus whose components are illustrated on the basis of the information
described in this paper.
and thus were equivalent to somatic nucleoli. This has been strongly
questioned by Callan and Lloyd (1960). They were unable to determine
which structures on the chromosomes corresponded to the peripheral
nucleoli.
With the electron microscope, these bodies have been shown to usually
have both granular and fibrous elements. The finding that these nucleoli
apparently undergo shedding of some of their contents is evidenced by
the strands extending from their periphery (Wischnitzer, 1958; Warten-