36 DUBLIN 



appearance of V's in the anaphase of the last spermatogonia! 

 division of Pcripatus, '99, he says, " This V-shaped approxima- 

 tion of the chromosomes into pairs is more than a mere coinci- 

 dence due to the crowding together of the chromosomes ; it is, 

 I think, the first sign of the pairwise union of chromosomes by 

 which the reduction of their number is effected and which re- 

 sults in the formation of bivalent chromosomes." But this V- 

 formation is, as he himself states, unusual at this point. That, 

 coupled with the fact that there intervenes between this and the 

 next appearance of the V's, a stage in which the chromatin is 

 closely packed into one dark staining mass, and that the indi- 

 vidual chromosomes cannot, with any distinctness, be made out, 

 throws some uncertainty on the result. In Pedicellina, on the 

 contrary, it is no difficult matter to so stain the young oogonia 

 and spermatogonia that at no stage do the chromosomes form a 

 close undecipherable mass. Thus Figs. 17-18 and 66 show that 

 while the chromosomes do group together, yet it is very appar- 

 ent that the individual chromosomes are in the form of V's, of 

 which there are about eleven. The main difficulty observed in 

 the evidence from Peripatits is thus overcome. 



Strong support to Montgomery's interpretation is given by 

 Sutton's observations on BracJiystola, '01, '02 and '03. This 

 author observed constant size-differences in the spermatogonia! 

 chromosomes and that these, with the exception of the single 

 accessory chromosome, were paired as regards size. This con- 

 dition persists throughout the eight successive generations of 

 spermatogonia, but is lost in the transition into spermatocytes, 

 in which the reduced number of chromosomes appears, but with 

 the same size -differences between the now bivalent chromosomes 

 that were so characteristic of the pairs of chromosomes in the 

 spermatogonia. ** These spiremes" (the reduced number), he 

 writes, "are graded as to size, just as were the chromosome 

 pairs of the spermatogonia." Further, " . . .a division may 

 be noted separating the spireme into two distinct limbs of ap- 

 proximately equal size, which are frequently doubled on each 

 other at the point of union." It is on this evidence alone very 

 difficult to disagree with Sutton that in the tetrads which later 



