66 EIGHTEENTH REPORT. 



which one can onh' surmise. Later on the thread shortens and 

 thickens and eventually the chromosomes appear in pairs, those of 

 each pair being so closely united as to give to each pair the appear- 

 ance of a unit. These double chromosomes are naturally hai)loid 

 in number as each consists of two chromosomes. In the first of 

 the two divisions that make up meiosis these pairs of chromosomes 

 split into their component whole chromosomes, one of which goes 

 to each daughter nucleus, so that the latter receives the haploid 

 number of whole chromosomes instead of the diploid number of 

 half chromosomes. The second of the two divisions is practically 

 normal. 



Although it is usually impossible to distinguish the chromo- 

 somes from one another in plant cells this is not true of all plants, 

 while in many animals chromosomes have distinctive shapes and 

 positions. In such organisms it has been possible to observe that 

 at meiosis one of each kind of chromosome goes to each daughter 

 nucleus. Since the pairs of such chromosomes arose by the union 

 of the gamete nuclei it is clear that the distribution of the com- 

 ponents of the pairs to the daughter nuclei at meiosis must bring 

 to the nuclei the corresponding chromosomes from the two gametes ; 

 i.e., the chromosome of male origin goes to one daughter nucleus 

 and that of female origin to the other. There is no reason to be- 

 lieve however, tliat all the chromosomes from one gamete go to 

 one daughter nucleus and those from the other gamete to the 

 other nucleus. Rather, it seems probable that the question as to 

 whether a given chromosome goes to one or to the other daughter 

 nucleus is wholly a matter of chance. 



We can now take a more comprehensive view of the subject of 

 sexual reproduction. It consists of the union of cells, with the 

 nuclear union as the most important feature. But, in order that 

 the process may be repeated, it involves also the reduction of the 

 diploid to the haploid number of chromosomes. This series of 

 events, then, the union of cells, the nuclear union and the reduc- 

 tion division form the sequence of processes that I call the Sexual 

 Cycle. 



Whatever group of plants or animals we study we find that the 

 sequence of events remains the same for the sexual cycle. On the 

 other hand there is the greatest variability as to the time inter- 

 vening between these different cardinal points of the cycle. 



In most animals fat least in most Metazoa) the nuclei of the 

 somatic cells are dii)loid in character, deduction division does 

 not occur until those cell divisions that produce the gametes (eggs 



