84 THE MEIOTIC PHASE [CH. 



maturation process is to produce germ-cells which again 

 have the reduced number. But it is obviously of great im- 

 portance to know whether the chromosomes unite indis- 

 criminately in pairs to give rise to the bivalents, or whether 

 the chromosomes differ among themselves and constitute a 

 definite series which must be handed on unchanged to the 

 next generation. If the chromosomes are all alike, it is 

 evident that any one may pair with any other, but if they 

 differ among themselves, one of each kind must obviously 

 go into each daughter cell if the two cells are to have 

 similar complements. In most animals it is quite easy to 

 see that the chromosomes differ among themselves in size 

 and in the shape which they assume in the metaphase and 

 anaphase of the division, and especially in insects it is often 

 possible to recognise each member of the series of chromo- 

 somes by means of careful measurements. It is then found 

 that in the somatic and spermatogonial or oogonial cells 

 there are two of each kind, so that these cells contain a 

 double set (whence the expression "diploid" or 2.n number), 

 and it is clear that the two sets are descended from the 

 chromosomes introduced into the zygote by the two germ- 

 cells which conjugated in its production. The diploid 

 number may thus be said to consist of similar paternal 

 and maternal haploid sets mixed together. When, as in 

 Lepidosiren, it is possible to see the chromosomes coming 

 together in pairs before the heterotype division, it is per- 

 fectly evident that similar or homologous chromosomes 

 unite to form the bivalents, but even when the union takes 

 place only in the spireme stage, and no separation and re- 

 union of the members takes place in diakinesis, it is equally 

 clear that each bivalent consists of two similar halves, for 

 the univalent components are exactly alike as they separate 

 in the anaphase of the heterotype mitosis. If, then, we call 



