A Small Chromosome iu Ascaris megalocephala. 
127 
small chromosome comes only Irom the male, that tlie upper parts 
ot‘ the egg- tubes, containing the oogonial divisions were not kept, 
so that I have them in only one worm, belonging to the same lots 
as those used for eggs. Here no trace of the small chromosome was 
tbund, although in the eggs of a worm from the same horse, mauy 
small chromosomes were seen. Oogonial divisions were studied iu 
two other worms, and the oocyte divisions in some preparations that 
Prof. Boveri had made some years ago, and no small chromosome 
was seen. The study of the oogenesis and spermatogenesis shows 
that a small chromosome in these stages is a verj- rare occurrence 
and seems never to he present in the oogenesis. 
The most feasible conclusion from these various observations ou 
the frequency of occurrence of the small chromosome seems to he 
that it is found iu too varying numbers, to make it probable that it 
has always a definite function, but that it must be due to a frag- 
mentation of one of the chromosomes of the fertilized egg. Frag- 
mentation might at times take place in later cleavages but the facts 
that the small chromosome has been found of .corresponding size in 
the different mitoses of the germ track (Figs. 12 and 13) and that 
it has the power to divide (Fig. 10) make it more probable that 
having once been formed in the fertilized egg, it keeps its individuality, 
at least tili the formation of the primary germ cells. As it has never 
been found in the divisions of the oogonia and oocytes, and so 
rarely in the spermatogonia and never in the spermatocytes, the 
question arises as to where and how it disappears. My material has 
thrOwu no light on this point. 
Size. 
The observations as to the size of the small chromosome lead 
to no more definite conclusions than those on its frequency. If it is 
a mere fragmentation, why should it so often be of the same size? 
In looking over preparations from one worm with a large percen- 
tage of eggs containing the small chromosome, the first Impression is 
that the size is constant (Figs. 1 and 2). When a series of plates are 
drawn and the chromosomes measured there is still found to be 
enough constancy to be remarkable, unless it he that the chromosome 
breaks most easily at the point where an end would be thrown off 
in a future diminution division. Another striking case of constant 
size is showu by figures 12 and 13, which represeut the third and 
