254 BIOLOGY 



the two nuclei once more dividing into two parts without any 

 splitting of the chromosomes, each of the four nuclei thus con- 

 taining two of the original chromosomes. Half of the nucleus 

 still within the egg is extruded, while the other half remains 

 within; Fig. 121 E, F. The nuclei which are thus extruded from 

 the egg are called polar cells, p, and have no further function, 

 since they have nothing to do with the individual which is to 

 arise from the egg. They are rejected products and soon dis- 

 appear. After the nuclei have divided the second time, the 

 nucleus remaining within the egg, with its two chromosomes, 

 once more passes toward the center of the egg and is called 

 the female pronucleus; Fig. G, f. The egg is now ready to 

 unite with the sperm. The egg, in other words, has become 

 mature, this process of the extrusion of the three small nuclei 

 being the essential feature of the process of maturation. 



The Origin of the Sperm (Spermatogenesis) . The origin of 

 the sperm is essentially similar to that of an egg, differing, 

 however, in one rather important point. As in the ovary, 

 the ordinary Cells in the sperm glands, during the early life 

 of the animal, continue their growth and division by the process 

 of simple cell division, with the normal method of the division 

 of the chromosomes. When, however, the sperms are about 

 to be formed, the cells of the spermary undergo a change simi- 

 lar to that described in the formation of the egg, except that 

 they do not materially increase in size. In each of these cells, 

 called a spermocyte (Gr. sperma = germ + cytos), the number 

 of chromosomes doubles itself, producing a number identical 

 with that found in the oocyte; Fig. 122 7. The chief difference 

 between this spermocyte and the oocyte at the corresponding 

 stage is that, whereas the egg has greatly increased in size by 

 the deposition of the food, the cell which is to form the sperm 

 does not increase in size. 



The next step in the development of the sperm is the divi- 

 sion of this cell into four parts. This step corresponds clearly 

 with the division of the egg cell into four parts, as shown in 



