378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



In either case the threads contract and thicken till they form dark- 

 staining bodies or masses of chromatin material. These bodies are 

 the chromosomes (C, Chr). The number of chromosomes is in gen- 

 eral constant for each species, there being in insect cells commonly 

 between 2 and 30 of them, though in some species there is a much 

 larger number. The chromosome number, however, is t3'pically not 

 the same in male and female cells of the same species, the male cells 

 usually having one less chromosome than the female cells. But in 

 some insects this rule is reversed, while in others the male cells have 

 as many chromosomes as the felnale cells, though in such cases one 

 is usually very small, as if in the process of being eliminated. The 

 different chromosomes in each cell of many species, moreover, have 

 characteristic individual shapes and sizes by which they can always 

 be identified and distinguished from one another. In the diagrams. 

 Figures 3 to G, and 9, three varieties of chromosomes are represented, 

 but these are conventional figures not intended to represent cells of 

 any particular species, and the reader must not get the impression 

 that chromosomes are necessarily of these forms. 



The division of the cell starts with the nucleus, and, indeed, with 

 the chromosomes themselves (fig. 3D), each of these bodies splitting 

 lengthwise into halves. Preceding this division of the chromosomes, 

 however, a minute sj^eck, called the centrosome (A, Cen) lying just 

 outside the nucleus, has divided and its two parts have moved to 

 opposite poles of the nucleus (B, C). The nuclear wall noAV dis- 

 appears and fine colorless lines radiate through the nucleus from 

 the two centrosomes, forming a spindle-shaped figure between them 

 (C). The chromosomes {Chr) become arranged across the middle 

 of the spindle (D) and then, as if pulled by a contraction of the 

 threads, their separated halves move to opposite ends of the nucleus 

 (E). Here they form two new nuclear groups, each of which 

 becomes surrounded by a new nuclear membrane (F). At the same 

 time, the cell itself constricts between the nuclei (E) and finally 

 separates into two daughter cells (F), each of which increases in 

 size and becomes in all respects a replica of the original cell. Growth 

 in all animal and plant tissues takes place by cell division, and most 

 cell divisions involve these complicated changes in the nucleus. 



We have seen that the original germ cells of any species ordinarily 

 contain the same number of chromosonies as the body cells of this 

 species. What, then, happens when two of them come together in 

 fertilization — is the number of chromosomes doubled? No; for in 

 the course of a few generations the number of chromosomes would 

 increase to impossible numbers. For a while, the germ cells mul- 

 tiply by divisions of the ordinary sort just described, resulting in 



