THE PROCESS OF CLEAVAGE. 55 



spermatic nucleus remain for a time separate, even after they have 

 approached each other. After a brief period of rest both of them 

 begin to exhibit simultaneously the changes which precede the for- 

 mation of the nuclear spindle. In each the chromatic substance is 

 metamorphosed into a fine thread, which is arranged within the 

 nuclear membrane in numerous windings. Each filament is there- 

 upon divided into two equally large coiled loops, the chromosomes 

 {fig. 25 cK). Now the two vesicular nuclei lose their delimitation 

 from the surrounding yolk, in which there arise at a little distance 

 from each other two polar corpuscles [centrosomes], surrounded by a 

 system of rays, which is at first faint, but subsequently becoriies 

 more distinct. Between the two centrosomes, the method of whose 

 development no one has as yet succeeded ; in obsei'ving, there are 

 formed spindle-fibres, and the four loops (chromosomes), set free by 

 the dissolution of the two nuclear membranes, so arrange themselves 

 that they lie upon the outside of the spindle at its equator. 



In the case of the egg of the Maw-worm, therefore, the union of the 

 two sexual nuclei, which terminates the act of fertilisation, takes 

 place only at the time of the metamorphosis to form the cleavage- 

 spindle, in which metamorphosis they take an equal share. In conse- 

 quence of this remarkable deviation from the ordinary course of the 

 process of fertilisation, van Beneden has been able to establish the 

 interesting and important fact that half of the chromosomes of the 

 first cleavage-spindle are derived from the egg-nucleus, and half from 

 the spermatic nucleus, and that consequently they may be distin- 

 guished as female and male chromosomes. Since in this instance, just 

 as in nuclear division ordinarily, the four loops are split lengthwise 

 and then move apart toward the two polar corpuscles (centrosomes), 

 there are formed two groups of four daughter-loops each, of which 

 two are of male origin and two of female. Each group is then meta- 

 morphosed into the quiescent nucleus of the daughter-cell". This 

 furnishes incontestable proof, that to each daughter-nucleus in each 

 half of the egg, which arises as the result of the first cleavage, there is 

 transmitted exactly the same amount of chromatic substance from the 

 egg-nucleus as from the spermatic nucleus. 



The first division is followed after a brief period of rest by the 

 second, this by the third, the fourth, etc., during which are repeated' 

 the same series of changes in nucleus and protoplasm that have just 

 been described. Thus in quick succession the 2 first daughter-cells 

 are divided into i, these into 8, 16, 32, 64, etc. (fig. 30), until 

 there has resulted a large spheroidal mass, which has received the 



