DEVELOPMENT OF THE OVUM 1075 



similar upon the whole to those seen in the mitotic or indirect 

 division of ordinary cells.* They have been most fully studied in 

 the eggs of certain invertebrate animals. 



The division of the cell is initiated by changes in the centrosome and 

 attraction sphere. The centrosome divides into two daughter centro- 

 somes. These take up a position one at each pole of the nucleus. 

 Each daughter centrosome is surrounded by a system of radiating lines 

 or filaments, which are less conspicuous than the chromatin filaments 

 of the nucleus, since they do not stain as these do. Meanwhile the 

 nuclear membrane and the nucleoli disappear, or at any rate become 

 indistinguishable from the rest of the chromatin skein. The skein 

 Breaks up into chromosomes, the number of which is constant for a given 

 species, but is not the same in all species of animals. 



The daughter centrosomes or astrospheres are united by meridional 

 achromatic fibres, which form a spindle running through the nucleus 

 from one pole to the other. The chromosomes arrange themselves at 

 right angles (equatorially) to the spindle, and then each chromosome 

 divides longitudinally into two. The halves of the chromosomes now 

 pass toward their respective centrosomes, being perhaps guided by the 

 fibres of the spindle. It results from this that two daughter nuclei are 

 formed, each with the same number of chromosomes as the original 

 nucleus, although with only half the amount of chromatin. The cyto- 

 plasm divides also, so that the parent cell is now represented by two 

 daughter cells. In ordinary cell division the two daughter cells are of 

 equal size, but in the division of the ovum which occurs before fertiliza- 

 tion the two resulting cells are very unequal. The large cell continues 

 to be known as the ovum; the small one is the first polar body. After 

 extrusion of the first polar body the ovum again divides unequally. A 

 new spindle forms, and a second polar body, again much the smaller of 

 the two daughter cells, is cast off. There is a difference, howeve*, 

 between the process of division which gives rise to the first and that 

 which gives rise to the second polar body. In the case of the latter a 

 so-called reduction-division occurs; the chromosomes do not split longi- 

 tudinally, but half of the original number pass into each daughter 

 nucleus. As to the significance of these changes there has been much 

 discussion. It is agreed that the result of the process is the expulsion 

 of a portion of the chromatin, the ovum now possessing only half the 

 original number of chromosomes, although nearly all the original 

 cytoplasm. In fertilization the original number is restored by the male 

 element when it arrives and penetrates the ovum. For in the final cell 

 division by which the mature spermatozoon is formed the chromosomes 

 of its nucleus are also, after two divisions essentially similar to those 

 occurring in maturation of the ovum, reduced to half the normal number. 



The two reduced nuclei in the fertilized ovum are spoken of as the 

 male and female pronuclei. By their union a single nucleus is formed 

 with the number of chromosomes normal to the species. 



An enormous amount of interesting work has been done with the 

 view of illustrating the connection of the complicated phenomena 

 described with the structure of the ovum. Only a bare reference 

 to one or two of the experiments is possible here. Driesch and 

 Hertwig find that the nucleus can be made artificially to change its 

 place with reference to the yolk, without hindering the development 

 * For figures illustrating the changes, see any good textbook of Histology. 



