iooqI SCHAFFNER— REDUCTION DIVISION IN AGAVE 199 



INVESTIGATION 



Incipient stages of division,— The sporogenous tissue is differen- 

 tiated very early in the young stamens and all vegetative divisions 

 come to an end lone before the earlier stages of the reduction division 



& MVXWXV. LL^ ^u,**x^x ov~ & 



are apparent in the microsporocytes. There is therefore no danger 

 in Agave of mistaking belated prophases of vegetative divisions for 

 stages of reduction. The nuclei of the incipient sporocytes are quite 

 small (fig. 1) and usually contain but one or two nucleoli and a rather 

 coarse chromatin net, in which are prominent dark-staining granules. 

 The cytoplasm is rather dense, with a spongy structure. As the 

 sporocytes grow in size the nucleus enlarges considerably, and at vari- 

 ous points in the enlarging net, masses of chromatin material appear 

 (fo 2) . Studied in detail the net reveals single chromatin granules 

 lying here and there in the linin meshes, and the clumps of chromatin 

 also show definite granules (fig. 2d). These masses do not appear 

 to be of a definite number, but approximate the reduction number of 

 chromosomes. They continue to become more conspicuous as the 

 early stages of division progress, until they have the appearance of 

 true prochromosomes. The meshes of the net at the same time 

 become larger, and the finer branches disappear, being probably 

 withdrawn into the larger threads and masses, like the pseudopodia of 



rhizopod. The linin network appears to be the active agent, the 

 granules being merely carried apart or together as the linin is moving. 

 1 think that there is no question but that these masses are the " pro- 

 chromosomes" of Overton and Strasburger. As stated, they are 

 approximately of the same number as the reduction number of 

 chromosomes. The evidence is strong that they represent pairs of 

 dividual chromosomes which are orienting themselves preparatory 

 to the formation of the spirem. Since the massing and lengthening of 

 these structures may not be synchronous, the apparent number need 

 «* necessarily agree with the reduction number. At this stage the 

 Jetoiite pairing of the individual chromosomes must occur, and it may 

 £ that this is the only definite pairing during the whole ontogeny. 



he number of prochromosomes may also appear greater than the 

 Ruction number, for the accumulation may at first be taking place 

 * two or more points of the paired chromosomes. It appears that 



C chro mosomes, extended and spread out like a rhizopod or an 



a 



