io Cutler, Parthenogenesis in Animals 



in the production of two daughter cells, each of which develops 

 into a spermatozoa, and each containing the reduced number of 

 chromosomes. 



In these insects therefore we have the peculiar feature that 

 the male passes the whole of it|s- existence with half] the normal 

 number of chromosomes in the germ cells. The difficulty of how 

 such an animal can at fertilisation restore the normal number to 

 the egg is overcome by the suppression of the reductive division 

 in the maturation of the male germ cells. In the somatic cells 

 the chromosomes may divide to form 32 or 64. 



Nachtsheim (191 3) reinvestigated the whole subject with great 

 care, and has in the main confirmed the conclusions arrived 

 at by Meves. In both fertilised and parthenogenetic eggs the 

 maturation divisions are the same. The innermost group of the 

 first polar body fuses with the seteond polar body to form the 

 " richtungskopulationkern;" while the outermost part of thel 

 first polar body degenerates. This " richtungskopulationkern " 

 is formed in all eggs, but it soon degenerates and gives rise to no 

 part of the insect, as Petrunkewitsch asserted. 



The whole process is rendered clear by reference to the table, 

 which is modified from the one given by Nachtsheim. (Diagram A.) 



Exactly comparable results have been obtained by other 

 observers in Osmia cornuta, -Xylocopa violarea, and in a few 

 species of ants. 



The question as to the way in which the sexes are produced 

 on the basis of sex chromosomes is also discussed by this author. 

 He assumes that of the female chromosomes thirty are somatic 

 and two sex chromosomes (30 -)- 2X). At maturation reduction 

 occurs, giving a nuclead complex 15-j-X, thus the unfertilised 

 egg will develop into a male. During spermatogenesis no reduc- 

 tion takes place and all the chromosomes divide, so that all the 

 spermatozoa possess 1 5 -)- X chromosomes ; thus fertilisation will 

 always result in the production of females. 



Rotifera. — The maturation of the parthenogenetic eggs of 

 Hydatina senta is similar, as regards male formation, to that 

 described above. According to Whitney, the parthenogenetic 

 eggs which will give rise ito females have only one maturation 

 division, and thus only one polar body is produced and the 

 chromosome number is not halved. In. the eggs which will give 

 rise to males, however, the two polar bodies are formed and fhe 

 chromosomes reduced; this also occurs in the maturation of the 

 winter egg. 



We are, unfortunately, ignorant of the spermatogenesis of 

 Hydatina, and therefore do not know whether the sj>erms are 

 all alike or whether there are twpi classes produced, one forrninjg 



