AN EXTRA DYAD AND EXTRA TETRAD IN CAMNULA 4195 
are the descendants of one primordial germ cell (Hegner, 714, ’15). 
If an extra chromosome arises through breakage or some irregu- 
larity in division at the separation of the primordial germ cell 
from the soma, it may, of course, pass into either of the daughter 
cells. If it becomes a part of the first germ-cell complex, it 
should be present in all the germ cells formed in the testis. In 
this case the chromosome count would be constant for the gonad. 
A supernumerary originating at the first division of the pri- 
mordial germ cell should be found in half the germ cells of the 
mature gonad. If this extra element arose through the breakage 
of a member of the normal chromosome group, one of the daugh- 
ter cells would have a deficient element in its complex. If non- 
disjunction of the two entire chromatids of a dyad were respon- 
sible for the appearance of the supernumerary, one daughter cell 
would lack altogether a member of some chromosome pair. As 
seems to be the case in certain instances of deficiency in Droso- 
phila (Bridges, *17), a cell with a deficient chromosome pair 
might not be viable. When such a cell is not viable, the animal 
should develop a gonad composed of half the number of follicles. 
But the work of Carothers (18, 717), Voinov (14), Wenrich (16, 
17), and Robertson (’15) on deficient elements; the haploid con- 
dition found in parthenogenesis (Schleip, ’08; Nachtsheim, ’13) ; 
the 55-chromosome females in a 56-chromosome race of Abraxas 
(Doncaster, ’14), the XO males in Banasa (Wilson, ’07 b), and the 
XO non-disjunction males in Drosophila (Bridges, ’16) demon- 
strate that a cell with a deficient chromosome pair may live. If 
it lived and fulfilled a normal réle in development, half the com- 
plexes found in the germ cells of the gonad should contain an 
unequal pair or should lack one member of a pair. 
The consequences of the mutation occurring at successively 
later stages in the germ line are obvious and are illustrated on 
the accompanying chart. If an abnormal division occurs, for 
instance, at the separation of a secondary from a primary sper- 
matogonium, the chromosome counts will be constant for the cyst, 
but not for the follicle. If it occurs at the division of a spermato- 
gonial cell, the counts will not even be constant for the cyst, and 
in the latter case, if complexes with an unequal pair or lacking 
JOURNAL OF MORPHOLOGY, VOL. 34, NO. 2 
