THE BIOLOGICAL BACKGROUND OF GENETICS 193 



Upon this linin network are strung numerous granules of a deeply 

 staining substance, or substances, called chromatin granules. Other 

 nuclear bodies, known as nucleoli, plasmosomes, and karyosomes, are 

 found in some nuclei, but they may be ignored so far as our knowledge 

 of their role in evolution is concerned. 



Chromosomes. — Far more important for our purposes than any 

 other cellular components are the chromosomes. Preparatory to the 

 process of mitosis the diffusely scattered granules of chromatin con- 

 dense into semi-solid masses, sometimes spherical or ovoid, sometimes 

 rodlike, sometimes V-shaped, etc. These are the chromosomes. Each 

 species of organism possesses in all its cells a certain definite number 

 and kind of chromosomes. Man has 48, Drosophila melanogaster has 8, 

 Oenothera lamarckiana has 14, Ascaris megalocephala has 4 in one varie- 

 ty and 2 in another, some crustaceans and some roses have over a 

 hundred chromosomes. Careful studies of the chromosomes of nu- 

 merous species of both animals and plants have revealed the significant 

 fact that, except in the males of some species, the numbers of chromo- 

 somes are even. Furthermore, it is possible to match them up in pairs 

 according to their sizes and shapes. The significance of this becomes 

 clear when we remember the fact that an individual starts out from a 

 zygote (fertilized egg) which is a composite cell to which each parent has 

 contributed one full set of chromosomes. 



Genes. — At this time it seems well to anticipate the later results of 

 genetic research to the extent of stating that each kind of chromosome 

 contains a unique series of hereditary units (genes). Certain genes lie 

 in certain chromosomes, and all the genes in a particular chromosome 

 are probably arranged in linear order like beads on a string. Since 

 the orderly arrangement of genes in bundles (chromosomes) and their 

 specific arrangement within a given bundle are both important in 

 heredity, it is essential that this specific organization be maintained 

 during development and reproduction. It is also essential for per- 

 sistence of type from generation to generation that the complete or- 

 ganization, not only of nucleus but of the cytosome, be maintained. 

 This is accomplished by that heredity mechanism par excellence, the 

 mechanism of mitosis. 



The central body. — Imbedded in the cytoplasm, usually in close 

 proximity to the nuclear membrane, lies a structure known as the 

 central body (centrosome). In most animal cells this structure is well 

 defined, but it is absent in most plant cells. The central body is pri- 

 marily associated with cell division, as will be seen later. 



