MICHIGAN ACADEMY OF SCIENCE. 65 



resulting inicleiis contains twice as many chromosomes as each of 

 the gamete nuck'i. \\'lien such a zygote nucleus divides each 

 chromosome divides iu the manner described above, so that each 

 daughter nucleus receives the double nund)er of chromosomes. 

 When we examine the chromosomes carefully in the dividing nuclei 

 of various insects and worms as well as of some plants we find 

 that the chromosomes appear to occur by twos. Thus in forms in 

 which there are in the gametes as many shapes of chromosomes as 

 these are in number we find in the nuclei coming from the division 

 of tlie zygote nucleus the same nundjer of shapes of chromosome* 

 but each shape represented by two chromosomes. Furthermore, the 

 two of a kind usually lie in close proximity to each other. We 

 must conclude that the corresponding chromosomes are equivalent 

 and that the component lumps of chromatin of which these corres- 

 ponding chromosomes are composed are also equivalent. Thus the 

 zygote nucleus and its progeny possess two chromatin masses of 

 every kind for every one in the gamete nucleus. 



Eventually, however, the time comes for new gametes to be form- 

 ed. If nothing new were to enter in we would expect these to 

 have double the number of chromosomes that were present in the 

 gametes of the previous generations, so that with each generation 

 the chromosome number would be doubled. Such a heaping up of 

 chromosonies is beautifully prevented by the complicated process 

 known as the reduction division or meiosis. This occurs wherever 

 sexual reproduction is found. By its means the number of chromo- 

 somes is reduced again from the ''diploid" to the original "haploid" 

 number. 



Tlie details of meiosis have not been worked out so completely 

 as to be free from controversy. Indeed, there are several theories 

 which differ radicall}^ but which are held by their advocates with 

 great tenacity. In objects so minute it is rather natural that pre- 

 conceived theories may influence the observations made by even the 

 most oi»en-minded of observers. Strasburger reports an amusing 

 instance of this. Two of the leading advocates of diametrically 

 opposite views as to the course of events in meiosis exchanged 

 their slides and all nmterial upon which they had arrived at their 

 conclusions. Each one, working with the other's slides and ma- 

 terial arrived, however, at his own original conclusions. 



In general the process seems to be a spinning out of the 



chromatin masses and the threads on which the}' are apparently 



strung into a verj^ slender tangled thread that compacts itself into 



a tight knot in which occur processes the nature and purpose of 



9 



