THE ONTOGENESIS OF VERTEBRATES 55 



worms, twelve in the mole-cricket, and sixteen in a water 

 beetle, the rat and Man, as well as in the pine and the 

 onion. Cyclops, a minute crustacean, possesses twenty-four, 

 as do also the frog, mouse, snail, lily and a fern (Osmunda). 

 The earth-worm has thirty-two chromosomes, the torpedo thir- 

 ty-six, and Artemia, a small shrimp, the unusual number of 

 1 68. Whenever a cell divides in a growing or proliferating 

 tissue, the maintenance of the same number of chromosomes 

 in each of the two resulting cells is effected by means of a 

 complex mechanism of minute threads, radiating from two 

 opposite centers, which results in the separation of each in- 

 dividual chromosome into equal halves, thus assuring for each 

 daughter cell, not merely the same number of chromosomes, 

 but halves of the same ones. This process is known as 'mito- 

 sis or karyokinesis. 



To the general rule concerning the constancy in the num- 

 ber of the chromosomes, there is, however, one very important 

 exception, and that is, in the germ cells, that become the 

 gametes in a conjugation, the starting point of a new organ- 

 ism. Here, owing to a difference in their mode of formation, 

 the number of chromosomes in a given species is exactly one- 

 half of that characteristic of the somatic cells of the same 

 species, and it is only by the fusion of the two gametes, ovum 

 and spermatozoon, that the normal somatic number is re- 

 stored. This reduction of the number of chromosomes is 

 brought about through an extremely complex process, the es- 

 sentials of which are:. first, the formation of certain germ-cells, 

 spcrmatogonium or oogoniiim, which develop twice the normal 

 number of chromosomes, and, secondly, two successive divi- 

 sions of the cells, and of the number of chromosomes also, by 

 means of which four cells are produced, 'each with one-half 

 the normal number. In the case of the male cells each of the 

 four is effective, and, through a metamorphosis in its form, 

 becomes a functional spermatozoon; but in the case of the 

 female, owing to the disadvantages which would arise from 

 the division of the yolk into four ova of equal size, one of 

 them retains it all and becomes a functional ovum while the 



