CYTOLOGY 37 



reticulum composed of chromatin. In this network are numerous knots 

 or lumps of heterochromatin. i\ttached to the reticukim are one, or some- 

 times two, nucleoli which differ from the chromatin in composition. 



3. Cell division is preceded by nuclear division or mitosis. The 

 reticulum is resolved into a number of separate threads, whose number is 

 fixed and characteristic for each species. These threads thicken and are 

 called chromosomes. Each of them has a distinctive shape or size. They 

 split lengthways, and the portions or chromatids separate and move to 

 opposite ends of the cell, where they reconstitute two daughter nuclei. The 

 chromosomes are drawn apart by two sets of cytoplasmic fibrils called the 

 spindle fibres, which appear only during nuclear division. The nucleolus 

 and the nuclear membrane disappear in the early stages of division and are 

 reformed at its conclusion. Between the daughter nuclei a cell plate is formed, 

 which becomes a new cell w^all (Fig. 18). 



4. Sexual reproduction consists essentially of the union of two nuclei, 

 one from each of the parents. Thus after fusion the nucleus contains two 

 sets of chromosomes, each pair of the same type forming what is called an 

 homologous pair. As this would inevitably lead to a progressive doubling 

 in the number of chromosomes, from generation to generation, there is a 

 periodic halving or meiosis of the chromosome number. The sexual nuclei 

 have always the half number (monoploid), and their union re-establishes 

 the double number (diploid). 



5. The special reduction division or meiosis does not always occur at 

 the same point in the life-cycles of difl:'erent organisms, but it is always 

 associated with the formation of some type of reproductive cell, either sexual 

 or non-sexual. Even in the latter case, which is common in plants, it may be 

 regarded as a preparation for eventual conjugation. The process of reduction 

 division involves two immediately successive nuclear divisions, resulting in 

 the formation of a quartet of cells. In the first division (heterotypic) the 

 chromosomes of homologous pairs unite together and then disjoin, each 

 chromosome being then separated to form part of a daughter nucleus. In 

 other words, at this stage ivhole chromosomes are segregated instead of halves 

 as in mitosis. The two daughter nuclei thus contain half the number of 

 whole chromosomes. In the second division (homotypic) each of these 

 chromosomes splits, as in mitosis, followed by separation as before. The 

 final result is four nuclei each with the reduced number of chromosomes. 



