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, 

 aud 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 cliroraosomes 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''), liowever, found only mitosis in 0. ranariim. Car- 

 noy has also seen transitional forms of division in the spermatic cells of 



