Mammalian ova, owing to an almost complete lack of yolk, are all small. The egg of a whale is about the size of a fern-seed, but the yolked eggs of birds are large--that of the great auk was 7.5 inches long. In man the ovum is from 0.18 to 0.2 mm. in diameter, scarcely visible to the {70} naked eye, and the spermatozoon is extremely minute. The human spermatozoon is only fifty-four thousandths of a millimetre in length, and from forty-one to fifty-three thousandths of a millimetre are taken up by its flagellum. The essential part is from four to six thousandths of a millimetre in length (Dr. L. N. Boston, _Journ, of Applied Microscopy_, vol. iv. p. 1360). A line of 18 human spermatozoa would reach only across the head of an ordinary pin. These spermatozoa have the power of locomotion in alkaline fluid. Henle found they can travel one centimetre in three minutes. The human ovum and spermatozoon are single cells, and the principal parts of a typical cell are the cytoplasm (called also the protoplasm), and, within this, the nucleus and centrosome. The centrosome is efficient in the process of cell-division. A few cells have also an outer envelope or membrane, and this part is well developed in the ovum. The nucleus is the centre of activity in a cell. In the resting state it is surrounded by a membrane, and within the membrane is an intra-nuclear network made up of chromatin and linin--the chromatin is an important element. The meshes of this network are probably filled with fluid. During the stages preparatory to the mitotic, or indirect, division of a cell into two cells (one of the methods of reproduction) the chromatin segregates in typical cases into two groups of loops, and each group has equal portions of the chromatin. When the chromatin is in this shape, a loop is called a chromosome. The chromosomes are very important. They occur in constant definite numbers in the somatic cells of the various species of many animals and plants, and it is probable that each species of plant and animal has its own characteristic number of chromosomes. Wilson (_The Cell in Development and Inheritance,_ New York, 1890) gives a list of 72 species in which the number has been determined. Man has probably 16 chromosomes in the somatic cell, and the mature male and female germ cells in man contribute eight chromosomes each to the nucleus of the impregnated ovum.