152 BULLETIN OF THE 
by Lowenthal (’89) in a Nematode (Oxyuris ambigua). It need hardly 
be said that amitosis in sexual cells is unexplained by any hypothesis 
yet offered regarding the biological significance of this type of division, 
and further investigations on this point are absolutely necessary before 
we can form any general opinion in regard to it. 
In the maturation and segmentation of the ovum no instance of direct 
division is known, and it is here that karyokinesis is exhibited in its 
most complete form. The well known observations of Boveri (’87) on 
the segmentation of the egg of Ascaris megalocephala are of special in- 
terest on this point. He found a modification of the chromatic threads 
as early as the two-blastomere stage, one of them (cell A) retaining the 
four chromosomes characteristic of the nucleus after fertilization, the 
other (cell B) undergoing a reduction of its chromosomes into the form 
of granules, The two blastomeres arising by division of cell A undergo 
the same differentiation, the nucleus of one (cell A?) retaining the 
chromatic loops, the other (cell A*) undergoing reduction, so that in 
the four-cell stage only one nucleus has retained its chromatic loops. 
The systematic reduction of chromosomes was observed up to the 64- 
cell stage. The important deduction Boveri makes from these facts 
is, that the cells retaining their ancestral nuclear characters are the 
Anlage of the sexual cells of the developing animal, and that the cells 
whose nuclei undergo a modification of the chromosomes are all somatic 
cells. In accordance with this hypothesis, the division of both male 
and female sexual cells ought always to be karyokinetic, and of a 
somewhat different type from the karyokinesis of the somatic cells of 
the same animal, The latter statement, indeed, holds true for the 
testicular cells of the salamander, as was discovered by Flemming (’87). 
It also appears from the work of Carnoy, that in the post-embryonic 
life of Arthropods mitotic division is of rare occurrence in the tissue 
cells, but is of constant occurrence in the reproductive cells of the same 
forms. 
As has already been stated (p. 147), attempts have been made to 
find a morphological connection between karyokinesis and direct divis- 
ion, and thus to solve the puzzling question of the relations they bear 
to each other. Carnoy (’85, p. 398) believes he has found transitions 
between them in the division of the numerous nuclei of Opalina rana- 
rum. Some of these show a distinct spindle, others none ; in both cases 
the nuclear membrane persists, and division is accomplished by constric- 
tion. Pfitzner (’86”), however, found only mitosis in O. ranarum. Car- 
noy has also seen transitional forms of division in the spermatic cells of 
