REPR OD UCTIOX. 453 



substance lying between them — either the cytoplasmic network or the achro- 

 matic substance of the nucleus — loses its reticular appearance, becomes fila- 

 mentous, and arranges itself in the form of a spindle with the threads extend- 

 ing from pole to pole (C, D). The groups of chromosomes become attached to 

 the spindle threads midway between the poles. At each pole there may lie a 

 centrosome, and about it the cytoplasm may become arranged in the form of 

 a star, the aster, though these structures are not universal among species. The 

 spindle with the two asters is known as the polar amphiastei', and the com- 

 plicated structure seems to be formed, as in ordinary cell-division, for the 

 sole purpose of dividing the nucleus into two portions. This is now per- 

 formed (E) : each quadruple-group of chromosomes splits into two, and these, 

 known as " double-groups," or " dyads," separate from each other and pass 

 toward the poles of the spindle. The nucleus is thus divided into halves. 

 While the division has been proceeding, the spindle has wandered halfway 

 outside the egg, and, when it is completed, one of the resulting nuclear halves, 

 comprising one-half of the full number of dyads, together with the centro- 

 some and the aster, finds itself entirely extruded from the egg and lying 

 within the perivitelline space. It is known as the first polar hod;/ ( /•'. p. 6 1 ). 

 The diminished nucleus within the ovum proceeds at once to undergo a second 

 mitotic division (G, H, I); each of the remaining dyads divides into two 

 single chromosomes, which are separated from each other ; and a second polar 

 body (p. b 2 ), containing one-half the number of single chromosomes charac- 

 teristic 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. In the mam- 

 mals no true tetrads are formed, and a considerable interval of time elapses 

 between the formation of the two polar bodies, during which the sperma- 

 tozoon enters the egg. But in them the process of maturation is the same in 

 essence as in the lower animals. In all species the chromosomes are reduced 

 to one-half the number belonging to the ovarian ovum ; in many species they 

 are then resolved again into scattered chromatic substance. Tin 1 nucleus 

 develops a membrane and again enters the resting stage. It is known hence- 

 forth as the egg-nucleus, or female pronucleus, and it await- the coming of the 

 spermatozoon. According to most observers, its centrosome gradually degen- 

 erates and disappears. 



Thus the curious process of maturation of the ovum is different in detail 

 from that of maturation of the spermatozoon. In the latter the spermatocyte 

 divides into four functional spermatozoa ; in the former the oocyte divides 

 into two functionless polar bodies (or, by subdivision of the first, three, which 

 have been called abortive c>j;<j;*) and one functional ovum. It is entirely 

 probable, however, that the essence of the process is exactly the same in the 

 two cases, and lies in the reduction of the number of the chromosomes. The 

 three important facts have now been demonstrated in a large number of 

 species, viz. — that in the maturation of both the ovum and the spermatozoon 



