238 Allen. — Nuclear Division in the 
as a separation of the original threads, which have been for a time in 
contact. Each of the parallel threads now contains a row of granules 
substantially uniform in size. In this stage in Myxine there is sometimes 
present also an indication of a second longitudinal split at right angles 
to the first. As a result of shortening and thickening, and of different 
degrees of separation of the daughter segments, the mature chromosomes 
appear variously as rings, 8’s. loops, V’s, X’s, or pairs of parallel rods ; they 
also often plainly show the second split. The separation in the first 
mitosis, as in the lily, is in the plane of the first split ; each daughter 
chromosome is two-parted as a result of the second split, and the two parts 
are separated in the next mitosis. 
The Schreiners accept Sutton’s notion of a pairing of homologous 
paternal and maternal chromosomes, and their subsequent separation in 
such a way that it is purely a matter of chance whether a particular 
chromosome shall pass to one or the other daughter nucleus. During the 
time of contact of the two chromosomes of a pair, there occurs a more or 
less intimate fusion (conjugation or copulation). These authors attempt to 
harmonize the facts of chromosome reduction in the Arthropods with their 
own observations on Vertebrates by assuming that in the former group 
there occurs first a parallel arrangement of two chromosomes, then a 
separation at one end, the chromosomes remaining attached at the other, 
so that when they are finally separated in the metaphases of one division 
or the other the appearance is that of a transverse separation. This 
supposition is supported by the various appearances of the chromosomes 
in Spinax. Here the most common form is that of a closed ring, resulting 
from the continued attachment of the chromosomes of a pair at both ends ; 
but a separation may occur at one end, leading to all possible variations in 
form from that of a ring broken at one point to that of two straight rods 
attached at one end and making various angles with each other. If the 
latter form should become common, the appearance of the chromosomes 
would be similar to that described for many Arthropods. 
De Vries, largely upon hypothetical grounds, has recently (’03) con- 
cluded that, shortly before the separation in the heterotypic division, the 
chromosomes become arranged side by side in pairs, the arrangement being 
such that each paternal chromosome is paired with a homologous maternal 
chromosome, and each paternal pangen, or group of pangens, is opposite 
the corresponding maternal structure. In this condition of intimate contact 
between the parental idioplasms, a mutual interchange of some of their 
pangens occurs, in such a way that when the chromosomes later separate, 
each set, whether paternal or maternal, contains a complete set of pangens, 
but some of the latter are paternal, some maternal, in origin. This inter- 
change of pangens is determined entirely by chance, so that, if the same 
process is repeated in a sufficiently large number of cells, every possible 
