THE EGG, FERTILIZATION, MATURATION, AND CLEAVAGE 29 



of polyspermy is too great, it may lead to disturbances that prevent 

 further development. These disturbances are usually due to disorgani- 

 zation of the maturation divisions by sperm which enter this region of 

 the egg. Such sperm may form uni-, bi-, or multipolar spindles or even 

 enter normal spindles to form multipolar figures. This condition may 

 or may not become adjusted. In binucleate moth eggs Doncaster (1914) 

 and Goldschmidt and Katsuki (1928) report that each nucleus fuses with 

 a sperm. Genetic evidence shows that the same is true for some binucle- 

 ate eggs of the wasp Habrohracon (Whiting, 1934). In no case is there 

 any evidence of a sperm functioning except after uniting with an egg 

 nucleus, but Huettner (1927) states that such is possible and shows that 

 it might conceivably be followed by normal development. 



Fertilization membranes are seldom mentioned for insects. In the 

 silkworm, Bataillon and Su (1931a, 1933) report that a strong fertiUzation 

 membrane is detached from the egg following induced parthenogenesis 

 and in the first brood of two-brooded stocks but not in the second brood 

 or in single-brooded stocks. The significance of this variation is not 

 known. 



The chromosome behavior during fertilization and maturation has 

 already been briefly described. As has been stated, the male of certain 

 animals produces two kinds of sperms differing visibly in chromatin 

 content; i.e., the male is heterozygous for sex, w^hereas the female pro- 

 ducing but one kind of egg is homozygous for sex. We have also shown 

 that although the autochromosomes occur in homologous pairs, the 

 accessory chromosomes in the male may either occur singly as an 

 x-chromosome (Fig. 3) or, if in pairs, as an x-chromosome and a y-chromo- 

 some; i.e., the male is heterogametic (digametic). The accessory chromo- 

 some is single in the Orthoptera, Homoptera, some Heteroptera (Protenor, 

 Hydrometra), some Diptera (Tephritis), some Coleoptera, and Neuroptera. 

 The diploid cells of the females of these forms have two similar x-chromo- 

 somes instead of one. At fertiUzation, eggs receiving an x-chromosome 

 will produce females; others receiving none will produce males. The 

 accessory chromosomes are double in the diploid cells of the males of 

 most Heteroptera, Diptera, and Coleoptera. In this case at fertilization 

 some eggs will receive an a:-chromosome and therefore produce a female, 

 whereas others that receive a ^/-chromosome will produce a male. In 

 some species of animals conditions are reversed, and it is the female that 

 is heterozygous for sex, as in certain Lepidoptera (Talaeporia, Fumea). 



More detailed accounts on this subject may be found in texts on 

 cytology (Doncaster, Sharp, 1934; Wilson, Schroder, 1928). Schroder's 

 text deals especially with cases among insects. 



Maturation. — In the insect egg maturation begins immediately after 

 the entrance of the sperm, so that the transformation of the head of the 



