314 APPENDIX B 



Similar facts have been reported by Wilson for several Insects 

 (seeJoiirn. Exp. Zool. ii, iii, 1905, 1906). 



Wilson finds constant size differences between pairs of chro- 

 mosomes, and either an accessory odd chromosome (which passes 

 into only one half of the germ cells) or a pair of idio-chromo- 

 somes of unequal size (one of which goes to one half, the other to 

 the other half of the spermatozoa), or both the accessory and the 

 idio-chromosomes (giving four kinds of spermatozoa). The idio- 

 chromosomes are supposed, again, to play a part in sex-deter- 

 mination. Several other observers have found these accessory 

 chromosomes, idio-chromosomes, and pairs of chromosomes of 

 different sizes in various Insects (Boring, Journ. Exp. Zool. iv. 

 1907; Stevens, ibid. ii. 1905, v. 1908 ; McClung, Biol. Bull. iii. 

 1902, ix. 1905; Montgomery, Biol. Bull., vi. 1904; Bamn- 

 gartner, Biol. Bull. viii. 1904-5 ; Zweiger, Zool. Anz. xxx. 1906 ; 

 Nowlin, Journ. Exp. Zool. iii. 1906) ; in Spiders (Wallace, Biol. 

 Bull. viii. 1904-5; Berry, Biol. Bull. xi. 1906); and in Myria- 

 pods (Blackman, Biol. Bull. v. 1903; Medes, Biol. Bull. 

 ix. 1905). 



It is a noteworthy fact that the accessory chromosome retains 

 its individuality in the resting stage (looking like a chromatin 

 nucleolus), while the others break up. The belief in the individu- 

 ality of these others rests therefore on the constancy of the re- 

 lative sizes from generation to generation. 



Further support for the hypothesis may be derived from theo- 

 retical speculations. We know that only n (one-half the normal 

 number) chromosomes are necessary for normal development 

 provided that they comprise a complete set. In sexual repro- 

 duction n maternal unite with n paternal. A study of the re- 

 ducing division shows that n whole chromosomes first pair with 

 and are then separated from n whole chromosomes, and that 

 when they differ in size those of the same size pair together, and 

 it looks as though paternal were here separated from maternal, 

 though the distribution of paternal and maternal to the two cells 

 will differ, almost certainly, in different cases. 



If the particles of which the chromosomes are composed are 

 also to be paired and separated, it would appear to be necessary 

 that their grouping should be constant, in other words that the 

 chromosomes should retain their individuality. 



(iii) A case of heterogeneous fertilization between eggs of Sea- 

 urchins and the sperm of Anted-on has been described above 

 (p. 262). Loeb has recently succeeded in rearing Plutei from 

 the eggs of Strongylocentrotntt fertilized by the sperm of a 

 Mollusc (Chlorostoma). Cytological details are not given (ArcJi. 

 Ent. Mec/t. xxvi. 1908). 



