500 
L. DONOASTER. 
factor produces maleness, a double dose femaleness. Tlie 
second hypothesis is preferable in not involving selective 
fertilisation, but involves certain other difficulties from which 
the former escapes. 
Essentially similar phenomena with regard to sex-chromo- 
somes have now been described not only in insects, but also 
in Myriapods (9), Arachnids (8, 62), Molluscs (70), Nematodes 
(11, 23, 45, 51), Birds (24, 24a), and Mammals (27, 56, 68, 
69). In all these cases the male is described as having a 
deficiency of one (or sometimes two) chromosomes, or as 
having a small one in place of a corresponding large one in 
the female. 
There are also a number of complications which have been 
observed in various cases, which will only be referred to 
shortly. In some species the male is described as having two 
or more chromosomes less than the female (66, 47, 48) ; in this 
case the two odd ones travel together into the same daughter- 
cell in the spermatocyte divisions, so that they may be regarded 
as behaving as one chromosome divided into two parts. In 
some cases both members of the pair may be compound, 
ddiis class of facts is possibly of importance in connection 
with sex limited inheritance. Cases of peculiar behaviour in 
heinnaphrodite species have also been described, the true 
nature of which is at present obscure.^ Two examples, how- 
ever, must be given, one of a hermaphrodite, the other of a 
parthenogenetic species, since these have been worked out 
rather fully and add very important facts to our knowledge. 
The chromosome-cycle of the Nematode Rhabdonema 
(Rhabditis) nigrovenosum has been investigated inde- 
pendently by Boveri (11) and Schleip (51), from whose 
papers the following account is combined. The species, as is 
well known, has alternate generations which are hermaphro- 
dite and bisexual. The hermaphrodites have as diploid 
number twelve chromosomes; their primary oocytes have six 
^ E, g. in Pteropoda, B, Zarnik, ‘ Verliaiidl. Deutsch. Zool. Gesellscli.,’ 
1911; summarised by Schleip, ‘ Ergehn. nnd Portschritte der Zool.,’ iii, 
1912, p. 250. 
