THE DETERMINATION OF SEX 99 



eggs which have only one maturation division ; l some of his 

 explanations of other exceptional cases, although ingenious, 

 will not now bear critical examination. One of the most 

 difficult is that of the hive-bee and those insects resembling 

 it, in which all eggs have two polar divisions and when ferti- 

 lised yield females, when parthenogenetic, males. Castle 

 supposed that the female determinant is extruded with the 

 second polar nucleus, leaving the egg male-bearing, and 

 accepted the observations of Petrunkewitsch, 2 who maintains 

 that the testis of the drone is derived from the fused polar 

 nuclei of the unfertilised egg. Since the second polar nucleus 

 by hypothesis contains the female determinant, the spermatozoa 

 may be female-bearing and cause the fertilised egg to be 

 female. But there is considerable doubt as to the accuracy of 

 the observation and in any case it cannot be applied to some 

 others ; e.g. Silvestri (see above) finds that in Litomastix 

 the polar nuclei are used up in forming the protecting 

 membrane of the embryos, and yet the sex-phenomena are 

 just as in the bee. An alternative speculation may be offered. 

 It is possible that while the female determinant is extruded in 

 the virgin egg with the second polar nucleus, the presence of 

 a spermatozoon in the egg may cause the male determinant to 

 be eliminated, leaving the egg-nucleus female-bearing. 3 This 

 would fall into line with the explanation suggested above of 

 gynandromorphic bees — that in them the sperm has conjugated 

 with the second polar nucleus. 



It now remains to describe work on the determination of sex 

 which has led to similar conclusions to those suggested above 

 but arrived at from a different starting-point. Castle suggested 

 that as sex is inherited as a Mendelian unit, it might at times 

 be " coupled " in the gamete with some other body-character. 

 It has been found that something of this kind actually does 

 take place in the case of certain varieties which are inherited 

 differently by the male and female. As an illustration of this 

 we will take some experiments made by the writer, accounts of 



1 E.g. Stevens, Journ. Exp. Zoo. ii. 1905, p. 313. But Morgan (Proc. Soc. 

 Exp. Biol, and Med. vol. v. 1908, p. 56) finds in Phylloxera that the females have 

 six chromosomes, the males five. And Stevens {Jour?j. Exp. Zoo. vi. 1909, p. 115) 

 suggests that in Aphis also one complete chromosome is extruded in the matura- 

 tion of male parthenogenetic eggs, but not in female eggs. 



2 Zool.Jahrb. xiv. 1901, and xvii. 1903. 



3 I find that this suggestion has also been made by Prof. T. H. Morgan. 



