REPRODUCTION. 891 



given rise to a large amount of discussion. Maturation occurs approximately 

 as the ovum is leaving the ovary, the exact time-relations being not yet deter- 

 mined. It consists of a karyokinetic division of the nucleus, essentially like 

 karyokinesis (mitosis) in ordinary cell-division, and an expulsion of one por- 

 tion from the cell. This occurs twice in succession. The cast-off bits of pro- 

 toplasm are known as polar bodies. The details of the process of maturation 

 are as follows (Fig. 308) : The nucleus of the original ovarian ovum contains 

 the same number of chromosomes as the ordinary tissue-cells (A). At the begin- 

 ning of maturation much of the chromatic substance begins to degenerate, and 

 later it disappears wholly (J5, C, D). The -remainder is rearranged into groups 

 of chromosomes, usually four in each group, which is called a " quadruple- 

 group " or " tetrad " (N). The number of tetrads is always one-half the num- 

 ber of original chromosomes, while the total number of chromosomes in the 

 nucleus at this stage is double the original number. The nucleus moves from 

 its position in the interior of the egg toward the surface, and the nuclear mem- 

 brane begins to disappear. At the same time the two minute cytoplasmic 

 structures, the centrosomes, which lie close beside the nucleus, separate and 

 take up positions at a considerable distance apart from each other, in some 

 cases even upon opposite sides of the nucleus. The substance lying between 

 them either the cytoplasmic network or the achromatic substance of the 

 nucleus loses its reticular appearance, becomes filamentous, and arranges itself 

 in the form of a spindle with the threads extending from pole to pole (C, D). 

 The groups of chromosomes become attached to the spindle threads midway 

 between the poles. At each pole lies a centrosome, and about it the cytoplasm 

 becomes arranged in the form of a star, the aster. The spindle with the two 

 asters is known as the polar amphiaster, and the complicated structure seems 

 to be formed, as in ordinary cell-division, for the sole purpose of dividing 

 the nucleus into two portions. This is now performed (_") ; each quadruple- 

 group of chromosomes splits into two, and these, known as " double-groups," 

 or " dyads," are drawn apart from each other and toward the spindle poles, 

 probably by contraction of the fibres of the spindle. The nucleus is thus di- 

 vided into halves. While the division has been proceeding, the spindle has 

 wandered halfway outside the egg, and, when it is completed, one of the result- 

 ing nuclear halves, comprising one-half of the full number of dyads, together 

 with the centrosome and the aster, finds itself entirely extruded from the egg 

 and lying within the perivitelline space. It is known as the first polar body 

 (F,p. 6 1 ). The r^'minished nucleus within the ovum proceeds at once to under- 

 go a second karyokinetic division similar to the first (6r, H, I)', each of the 

 remaining dyads divides into two single chromosomes, which are pulled apart 

 from each other; and a second polar body (p. 6 2 ), containing one-half the 

 number of single chromosomes characteristic of the tissue-cells, is extruded. 

 Apparently the two polar bodies are of no further use. In many animals the 

 first divides into two, but sooner or later both degenerate and disappear. The 

 remnant of the nucleus left within the egg, much reduced in size, wanders 

 back to the interior. Its chromosomes, reduced to one-half the number 



