The Idiochromosomes in Ascaris megalocephala and Ascaris lumbricoides. 425 
plasniic tkrcad breaks, and each idiocliromosonie is drawn dose up to 
its own group of chromosomes (pl. XXI, fig. 12). In worin B, most often 
in this first maturation division, the idiochroniosome passes nndivided 
to one of tlie secondary spermatocytes (pl. XXI, fig. 13), althoiigh it is 
sonietimes didded and distributed just as described for worm A. 
As the dyads bcgin to divide in the second maturation division, the 
dumbbell-shaped idiochroniosome lies between them (pl. XXI, figs. 14 — 15), 
with each end drawn out into a protoplasmic connecting fibril (pl. XXI, 
figs. 14 — 15). The idiocliromosonie approaches one pole (pl. XXI, fig. 17), 
the protoplasmic precess breaks at the other (pl. XXI, fig. 18), and the 
division of the cytoplasm is completed, with the idiochromosome in the 
one spermatid (pl. XXI, fig. 19). As in the first maturation division so in 
this, when the daughter cells are artificially separated the protoplasmic 
connecting thi’cad may be pulled out without breaking (pl. XXI, fig. 20). 
In worm B the two halves of the idiochromosome which at the end of 
the first maturation division lodged undivided in one of the two secon- 
dary spermatocytes, are distributed in the second division to the daughter 
spermatids (pl. XXI, fig. 21). Thus, in either case, one-half of the sperma- 
tozoa contain the two well-known rod-formed elements while the remai- 
iiing spermatozoa possess, in addition to the two large chromosomes, the 
small heterotropic idiochromosome. 
Borixg (1909) gives only one case (pl. X, fig. 8) of a small chromo- 
some in an egg of univalens, but Boveri (1909) interprets this as due to 
fragmentation from one of the large chromosomes, as he does also one 
of the two small elements found by himself in a fertilized egg of bivalens. 
Boveri (1890) had noted two small intensely-staining spherical bodies 
lying near the two tetrads of the first polar spindle, and now (1909) he 
believes them to be the two idiochromosomes of the female disconnected 
from their usual adherence to the large chromosomes. In a univalens 
worm I have found a niimber of equatorial plates of the first cleavage 
division which show an unpaired idiochromosome (pl. XXI, fig. 22). 
Ascaris lumbricoides. 
In Ascaris lumlricoides the sex-determinant is in the form of a group 
of five univalent idiochromosomes. In the equatorial plates of the sper- 
matogonial divisions there are forty-tliree chromosomes (pl. XXII, fig. 23). 
Düring synapsis thirty-eight of these elements combine to form the nine- 
teen bivalent chromosomes, while lying toward one side of the vesicular 
nucleus is the differential group of five idiochi'oniosomes (pl. XXII, fig. 24 i). 
