The chromatin thread next becomes shorter and thicker, the nucleoli begin to disappear, and the thread breaks up into a number of segmentschromosomes-which vary in number in different species, but are fairly constant in the same species (fig.
The chromatin is distributed throughout the cytoplasm in the form of granules which may be regarded as a distributed nucleus corresponding to what Hertwig has designated, in protozoa, chromidia.
The Structure of the Nucleus.In the living condition the resting nucleus appears to consist of a homogeneous ground substance containing a large number of small chromatin granules and one or more large spherical granulesnucleolithe whole being surrounded by a limiting membrane which separates it from the cytoplasm.
He thinks it may possibly originate in the vacuolization of the central region, and the accumulation of chromatin granules therein.
Oertel finds an explanation of this want of complete celldifferentiation, loss of function, and acquired vegetative activity in the non-homogeneous character of the nuclear chromatin elements of the cell, and maintains that the different properties of the cell are carried and handed down by the different orders of chromatin loops.
It forms a part of the 1mm or plastin network of the nucleus and may become impregnated with varying quantities of chromatin stored up for use in the formation of the chromosomes and other nuclear activities.
The chromatin is practically identical with nuclein.
It is clear, however, that an equal quantitative division and distribution of the chromatin to the daughter cells is brought about; and if, as has been suggested, the chromatin consists of minute particles or units which are the carriers of the hereditary characteristics, the nuclear division also probably results in the equal division and distribution of one half of each of these units to each daughter cell.
As they pass into this position they undergo a longitudinal splitting by which the chromatin in each chromosome becomes divided into equal halves.
That these granules consist of a material similar to the chromatin of the nucleus of higher forms is very doubtful, and the comparison with the nucleus of more highly organized cells rests on a very slender basis.