342 Kansas Academy of Science. 



previous plane of union or not. Then in a short time the two 

 spireme divide transversely, shown in D, into segments termed 

 chromosomes. 



In the mother germ-cells of many species of plants and ani- 

 mals, the two spirem threads divide transversely, before unioD, 

 into chromosomes. These unite, chromosome with chromosome, 

 chromomere with chromomere, remain in close union for several 

 days, and then separate, as do the spirem threads. From either 

 method of union and division there always results a definite num- 

 ber of chromosomes peculiar to the species. In this case the num- 

 ber eight has been taken, shown in D. Every species of plant and 

 animal has- its peculiar number. In certain red algse there are 40 

 chromosomes; in sharks, 36; in certain gasteropods, 82; in the 

 salamander, trout, mouse, fern, lily and man, 24; in the sagitta, ox, 

 guinea-pig and onion, 16; in the grasshopper, 12; in a liverwort, 

 8; and in some of the nematodes, 8, 4 or 2. 



The mother egg and mother sperm-cells of the same species of 

 plant or animal have the same number of chromosomes in their 

 nuclei. These mother germ-cells difPer in appearance chiefly in 

 their form and in the amount of cytoplasm they contain, the mother 

 egg having even many thousand times as much cytoplasm as the 

 mother sperm. 



In animals before the egg and sperm are ready to conjugate the 

 mother egg and the mother sperm nuclei must each reduce the num- 

 ber of chromosomes one-half. In the mother eg^-cell, as shown in ^ 

 and F, the nucleus has divided unequally by mitosis and four of 

 the chromosomes have passed out into the cytoplasm with a small 

 part of the nucleoplasm, becoming the first polar body, E-^. Actu- 

 ated by a very primitive instinct, the chromosomes of the nucleus and 

 the polar body split, and each nucleus divides by mitosis, the for- 

 mer unequally as before, and the latter equally. The result is 

 shown in F. In a similar way the mother sperm-cell divides by 

 mitosis into daughter sperm-cells and these into granddaughter 

 sperm-cells or sperms, again making four resultant cells according 

 to the primitive instinct. It will be noted in G and ZTthat the 

 mother sperm-cell divided equally, making four functional sperms, 

 while the mother egg-cell, by dividing unequally, produced only 

 one functional egg. The three polar bodies may perhaps assist in 

 organizing the cytoplasm for the work of preparing nutrition for 

 the embryo. 



In plants the process of maturation is the same as in animals, 

 except that the resulting cells are not eggs and sperms, but spores, 



