120 THE BIOLOGY OF INSECTS 



secondary oocytes are extremely unequal in size, one of 

 them, the maturing egg, keeping nearly all the cell- 

 protoplasm and yolk, while the other is a minute *' polar 

 body " (Fig. 32, c). This division is followed by a second in 

 which (as in the formation of spermatids) each chromosome 

 is split. The egg, now mature, again keeps nearly all the 

 cell-substance, though its nucleus corresponds to that of a 

 spermatid or spermatozoon, while its sister- nucleus, sur- 

 rounded by a small mass of protoplasm, forms a " second 

 polar body." If, as sometimes happens, the first polar 

 body divides into two daughter- cells it is clear that there 

 are four reduced nuclei (corresponding to the four mature 

 sperm-nuclei), but only one of them becomes the nucleus 

 of a mature egg ; the other three have cell-bodies so minute 

 that they can perform no reproductive function, and they 

 were formerly regarded merely as " extrusions " from the 

 ripening egg. The nucleus of the mature egg has how- 

 ever been reduced, so that when the sperm-nucleus enters 

 at fertilisation, the conjoint or zygote-nucleus becomes 

 quantitatively double that of either gamete and the number 

 of chromosomes is restored to that normal for the body- 

 cells of the creature to be developed from the zygote 

 (Fig. 32). In very many insect eggs the polar-nuclei 

 remain within the egg-substance, or the polar bodies after 

 extrusion are reabsorbed by it. 



These processes connected with maturation and fertilisa- 

 tion, the more essential features of which have been briefly 

 sketched, have become known through investigations carried 

 out during the last half-century. They were first elucidated 

 by E. van Beneden (1883), T. Boveri (1899), and many 

 students of various animals other than insects ; full details of 

 these and subsequent discoveries may be found in the recent 

 text-book of E. B. Wilson (1925). It is evident from the 

 appearance of a definite number of chromosomes in succes- 

 sive generations of creatures of the same kind that there is 

 real individuality and continuity in these bodies and that 

 they are closely connected with the transmission of inherited 

 characters. Half the chromosomes present in the zygote- 



