40 DUBLIN 



phenomena in these groups where the processes are most fully 

 known fall in line with the conceptions of Montgomery and 

 Sutton. 



C. Post- Synaptic Processes. 



The reduction once accomplished, the chromosomes in the 

 form of V-shaped bivalents show again marked plasticity. The 

 arms of the V's split longitudinally, producing two sister struc- 

 tures united at several points along their course, and then become 

 continuous through the increase of the angle to i8o°. In this 

 way are produced the parallel bivalent threads, like those in the 

 Copepods or when the ends unite, the elongated rings described 

 in Gryllotalpa {yovci Rath, '92). This extension of the angle is, 

 in reality, no new process. Changes very similar were described 

 by Vom Rath in Gryllotalpa and later by Paulmier in Afiasa — 

 cases from which that of Pedicellina differs only in that the 

 changes are somewhat more pronounced. The figures of 

 Hacker, moreover, point strongly to the probability that the 

 parallel bivalent threads of the Copepods are also so formed. 

 By the final concentration, these two types finally become in 

 Pedicellina the rings and the elongated bars of the prophases of 

 the first maturation division. But in either case, the structures 

 are equivalent to tetrads, the longitudinal split (most clearly 

 identified in the double bars) separating sister bivalents. We 

 are thus presented with a complicated series of changes, in 

 proper sequence, all leading to the formation of such a struc- 

 ture that its various parts may be distributed in the ensuing 

 maturation divisions to different cells. 



In the formation of the longitudinally split bivalent chromo- 

 somes, Pedicellina throws light on the whole problem of chro- 

 mosome-development in the growing ^<g^ and sperm. Two 

 types have been recognized to exist, as regards the origin of 

 these bivalent chromosomes. In the first, a continuous spireme 

 is said to arise in the earliest oocytes, often already longitudi- 

 nally split, which later segments into one half the normal num- 

 ber of double bars {^Copepod type, Hacker). In the second, a 

 continuous spireme is never formed. The reduced number of 

 bivalent chromosomes appears at the very beginning in the form 



