STUDIES ON CHROMOSOMES 65 



d. The Growth-period and Maturation-prophases 



The foregoing facts demonstrate in the clearest manner that 

 this individual of M. femoratus differs from all other individuals 

 of the genus heretofore examined, with the exception of Mont- 

 gomery's material of M. terminalis, in having an odd or unpaired 

 idiochromosome (accessory chromosome) which corresponds to 

 the larger member of the pair of unequal idiochromosomes found 

 in other individuals. They show also that the third small chro- 

 mosome is not a small supernumerary of the -type found in other 

 individuals, and is nothing other than a third m-chromosome. 

 This is fully borne out by the growth-period and prophases. As I 

 have indicated in earlier papers, the m-chromosomes are in general 

 characterized during the growth-period by the fact that they re- 

 main univalent (there are some exceptions to this) and in most 

 cases (of which Metapodius is one) are in a diffuse and light- 

 staining condition. Further, as was first shown by Gross ('04) in 

 Syromastes, as a rule they only conjugate to form a bivalent in 

 the final prophases of the first division very often not until the 

 spindle is formed and the chromosomes are entering the equator- 

 ial plate. Such a late prophase, from a 22-chromosome individual 

 of the same species (No. 29) is shown for the sake of comparison 

 in fig. 2c, the two separate m-chromosomes appearing above 

 and towards the left. Their final conjugation always takes place 

 at the center of the group (fig. 2j, 3, e-g). 



In individual No. 64, prophases of every stage are shown in 

 hundreds of nuclei. In the latest stages, after the nuclear wall 

 has broken down, three separate small chromosomes are shown 

 (fig. 2a) which may be seen coming together in the final prophases 

 to form the small central triad. Figs. 1m, n show two earlier 

 stages from the same cyst with the last, one of them showing the 

 beginning of the spindle-formation, the other an earlier stage 

 when the asters are very small and often invisible. Each of these 

 shows the three separate small chromosomes, as before. At this 

 time all the chromosomes are compact and deeply stained. In 

 still earlier stages, at a time when the bivalents are all diffuse and 

 appear in the form of lightly staining double crosses, rods, etc., 



THE JOURNAL OP EXPERIMENTAL. ZOOLOGY, VOL. 9, NO. 1. 



