The discovery that a particular pair of satellited chromosomes is concerned in producing the nucleolus in telophase depended an several independent lines of observation. The satellites were first observed by S. Navashin in 1912 as bodies attached to the nucleolus, and were believed to become attached to their respective chromosomes later in the prophase.
The earlier cytologists believed the nucleolus to arise as an independent body in telophase and to pass material an to the chromatin thread in prophase. A definite connection of morphological significance was first observed in polten mother cells of Lathyrus by Latter in 1926, between the nucleolus and the chromatin thread, the latter being constantly attached in leptotene and zygotene stages to a more deeply staining portion of the nucleolus which was called the nucleolar body.
In 1931, from observations of somatic telophase in various plants, Heitz concluded that a particular pair of satellited chromosomes was responsible for producing two nucleoli, or one if they were lying near enough together to touch and fuse.
In 1934, from a reciprocal translocation in maize in which the nucleolar body was broken into two, McClintock concluded that the nucleolar body was situated at a particular locus of the chromosome adjacent to the satellite, and that this body was normally concerned in organizing the nucleolus from the material of the chromosome matrix.
In 1934, essentially the saure relationships were shown for animals by Dearing, through an investigation of the somatic nuclei of Amblystoma. Two subterminal granules an the split telophase chromosome were found to fuse shortly, afterwards growing gradually into a nucleolus from that chromosome, which bears a large satellite at metaphase.
Among subsequent observations, it is found that in the polten mother celis of rice a terminal knob rather than a satellite is attached to the nucleolus, and that this attachment remains while the nucleolus gradually disappears.
Primula seclusa and certain strains of
Narcissus bulbocodium. var.
genuinus near Coimbra also appear to lack satellited chromosomes, yet produce a nucleolus. Similar conditions were found in Liverworts. The presence of a connecting thread is therefore unessential.
A diploid nucleus in plants or animals then usually contains a pair of chromosomes which are mainly concerned in organizing the nucleolus in telophase at a particular locus. This locus is usually subterminal, giving a satellite attached by a thread, which may increase greatly in length as the nucleolus grows.
Various other conditions are derivable from this through translocation, or perhaps in some Gases through mutation. Questions of variation in the size of satellites, or their disappearance, and their relation to particular phenotypic characters require further elucidation.
There appear to be two kinds of satellites, one concerned in the production of a nucleolus and the other not. Why the nucleolar body should be generally subterminal an a chromosome is not clear, nor is the nature of the delicate thread connecting the satellite to the chromosome.
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