A Small Cliromosome in Ascaris megalocepliala. 
129 
Figure 20 is auotber case probably due to fragmentatiou ; it is an 
egg of a nnivalms worin wbere tbe cbromosonies of tbe first cleav- 
age were often hook-shaped at tbe end as in figure 4. Prof. Boveki 
bas allowed me to use bere a drawing of tbe first cleavage in uni- 
valens which he made some time ago but bas uever publisbed. Figure 21 
is a diagram made from bis drawing, representing only one of tbe 
two chromosomes, in early anaphase. Tbe small ehromosome at one 
side is bere evidently a piece of one of tbe daughter ebromosomes. 
This shows bow tbe small ehromosome may arise. It must, however, 
usually originate earlier, as it so often appears in tbe equatorial 
plate of tbe first cleavage. 
Conclusions. 
The observations described in tbis paper can give no definite 
answer to tbe question wbether tbe small ehromosome found so fre- 
quently in Ascaris is a ehromosome unit in itself, or a fragment of 
one of tbe long ebromosomes. Some observations point to its being 
a ehromosome unit; i. e., it occurs frequently in a large proportion 
of tbe eggs, there is a general constancy in tbe size in any one 
worm; and it usually appears singly. Other observations seem to 
prove it to be a fragment; i. e., it is entirely absent in some worms; 
tbere is Variation in size in spite of a general constancy; it is at 
times found near tbe end of a markedly short ehromosome ; some- 
times two are present. There is tbe third possibility, that we are 
dealing bere with two different things, a ehromosome 
unit and a fragment. Herla bas already suggested that tbe 
small ehromosome may be due to two causes, but he uiakes tbe two 
causes, fragmentatiou and double fertilization , fragmentatiou wbere 
tbe small ehromosome is so evidently a part of one of tbe longer 
ehromosome, and double fertilization wbere tbe long chromosomes and 
tbe short one seem complete in themselves. That part of tbe cases 
are due to double fertilization is utterly untenable, for tbe small 
ehromosome is too short to be a univalens ehromosome and tbe 
cleavage would not take place normally if four asters were present 
as we find in disperm eggs. But there is tbe other possible expla- 
nation for cases not evidently due to fragmentatiou, tbe presence of a 
definite ehromosome unit with some definite function, such as sex 
determination. If tbe small ehromosome in Ascaris is ever a sex 
determinant, we must make many large assumptions. A sex-deter- 
Archiv f. Zellforscliang. IV. 9 
