THE CELL AND ITS CONSTITUENT STRUCTURES. i6i 



tion eventually takes place. Owing to the previous longi- 

 tudinal fission and subsequent contraction, the position of 

 each of the duplicate chromosomes is marked by the pre- 

 sence of four spheres. During the first mitosis these are 

 distributed to the daughter nuclei in pairs. During the 

 second division the two spheres in each couple separate in 

 like manner, so that after the two divisions each of the four 

 nuclei possess one of the original four spheres. Thus, in 

 this case we are not merely dealing with a distribution of 

 symmetrical quarters of one chromosome, but with a distribu- 

 tion of halves of two enth-e chroinosonies ; that is to say, if 

 we for the moment adopt Weismann's standpoint, the here- 

 ditary qualities in this case are divided equally between the 

 pair of nuclei which have received halves of dijf event chro- 

 mosomes, whereas if the reduction had been followed by 

 two longitudinal divisions of one chromosome, all the here- 

 ditary qualities would have been distributed to each of the 

 four nuclei. 



It is clear that the former method of division, that 

 described by Riickert for Copepods, exactly fulfils the 

 requirements of Weismann's theory by removing one half 

 of the material entities, in which he believes the hereditary 

 qualities to reside, from each of the sexual cells. But it is 

 obvious that, in order to admit the general validity of a re- 

 duction division such as seems to be warranted by a study 

 of Crustacea, we ought to find essentially the same type of 

 chromosome distribution recurring everywhere. Evidence 

 is not, however, wanting to prove that by no means all 

 organisms do conform to the crustacean type in these im- 

 portant respects, and even the most violent efforts have so 

 far failed to fit them to this Procrustes bed. 



In the divisions which result in the formation of the 

 spermatozoids in Ascaris niegalocepkala, Brauer ^ discovered 

 that, whereas in the antecedent mitosis, four chromosomes 

 appeared in the nuclear spindle, in the first of the two divi- 

 sions we are now considering, only two chromosome rudi- 



^ Brauer, " Die Spermatogenese von Ascaris megalocephala," Arc/i. 

 f. Mikr. Afiat., Bd. xlii. 



