502 
L. DONCASTEK. 
doubles, the mature egg six singles. The spermatogonia have 
twelve, the spermatocytes five doubles and two singles; 
the singles do not divide at the second spermatocyte division, 
and one of them gets left on the spindle, so that half the 
spermatids have six, half have five.^ Half the fertilised eggs 
thus have twelve, and become females; half have eleven, and 
become males. Iii the gametogenesis of the free-living 
bisexual forms, all mature eggs contain six ; in the males, 
the odd one does not divide at one spermatocyte division, so 
that half the spermatids have six and half five. Those with 
five appear to degenerate, so that all functional spermatozoa 
have six, and thus all fertilised eggs have twelve and become 
hermaphrodites. Why, in one generation, individuals with 
twelve become females, in the next, hermaphrodites, is not 
explained. It is noteworthy that the absence of one chromo- 
some is not the cause of the production of spermatozoa rather 
than ova, since both oogonia and spermatogonia of the herma- 
]dirodite have twelve. 
The second special case is that of the Aphids and Phyllox- 
erans described by Morgan (35-37) and von Baehr (3). 
Morgan’s account of Phylloxera caryaecaulis maybe 
taken as an example. The stem-mother ” has six chromo- 
somes; her parthenogenetic eggs have no reduction and con- 
tain six. The parthenogenetic females produced from these 
eggs are of two kinds, which may be called female-producers 
and male-producers. Each form contains six chromosomes, 
but the male-producer has five large and one small, the female 
producer six large. Morgan suggests, without actually 
proving it, that the cause of this is that one of the chromo- 
somes is compound, and part of it is extruded without division 
at the polar mitosis of the egg which gives rise to a male 
producer.- The male-producing female lays small eggs which 
^ This is Schleip’s account of the second si^ermatocyte division ; 
according to Boveri either both the “ accessories” may be included in 
one spermatid or one may go into each, thus giving spermatids either 
with five and seven, or with six chromosomes. 
- In ‘ Heredity and Sex ’ (Xew York, 1913, p. 182), Morgan suggests 
that the difference between female-producer and male-producer may be 
