

i I22 REPRODUCTION 



develops at each wound. Impregnation does not appear to be a 

 necessary factor, nor even contact of the ovum with the uterine 

 mucous membrane. On the other hand, ovulation, the discharge of 

 an ovum or ova, or at any rate the condition of the ovary associated 

 with this discharge, seems to be indispensable. For extirpation of 

 the ovaries in a large number of guinea-pigs prevented the formation 

 of deciduomata from wounds of the uterus made at the most favour- 

 able period after copulation. The uterus then appears to have an 

 inherent power of responding to such a stimulus as a mechanical 

 injury by the production of a decidual structure, but only under the 

 influence of the ovary. The ovarian factor is probably not nervous 

 but chemical, some specific substance which acts on the uterus 

 being liberated periodically in connection with the sexual rhythm. 

 Development of the Ovum. Before fecundation, and apparently 

 as a preparation for it, the ovum is the seat of remarkable changes, 

 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, however, 

 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 

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



