REPRODUCTION AND HEREDITY 117 



cell-divisions in which the cell-nuclei take an important 

 part. The microscopic examination of suitably stained 

 sections reveals in the nucleus of any cell the presence of a 

 substance (chromatin) which takes up the microscopist's 

 stains very readily, and therefore becomes conspicuous. 

 The chromatin often appears in the form of knotted or 

 looped threads, but when a cell is going to divide it becomes 

 segregated in a definite number of minute ovoid or rod- 

 like bodies (chromosomes) each of which splits into two 

 halves — " daughter- chromosomes " — one of these going to 

 form part of the nucleus of either of the '* daughter- nuclei " 

 resulting from the cell-division (Fig. 32, a). As the growth 

 and repair of all the tissues of the body depend upon an 

 enormous series of such cell- divisions it follows that the 

 number of chromosomes must remain constant throughout 

 the body-cells of any creature, and in a large number of 

 insects (as in other animals) a definite number is character- 

 istic for each kind of creature. A full account of the forms 

 and behaviour of these bodies may be found in the work of 

 L. Doncaster (1920) or E. B. Wilson (1925) ; only the most 

 essential points can be discussed here, but the student of 

 Insect Biology may profitably remember that a vast amount 

 of information about cell-structure and behaviour which 

 has an important bearing on the Hfe and development of 

 animals generally, has been obtained through the study of 

 insect tissues and germ-cells. 



Now, we have seen that in the fertilisation of an egg, 

 the essential process is the conjugation of the two germ- 

 nuclei, egg- nucleus and sperm-nucleus. The number of 

 chromosomes in the zygote-nucleus of the fertilised egg 

 must be clearly the sum of the numbers in the two gamete- 

 nuclei. But if these numbers were the same as in the body- 

 cells of the insect, they would be necessarily doubled after 

 every reproductive pairing, they w^ould be repeatedly 

 doubled throughout successive generations and the creature's 

 nuclear constitution would become impossibly complex. 

 This condition is prevented and the number of chromo- 

 somes is kept constant through successive generations by 



