6 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



repeated normal mitotic division cease dividing and have a period of 

 growth when they are known as "spermatocytes." They then undergo 

 a maturation period during which two maturation divisions rapidly 

 succeed each other (Fig. 3B). The first maturation division results in 

 two spermatocytes of the second order. These immediately divide again 

 to form four spermatids from each spermatogonium. Each spermatid 

 develops without again dividing into functional spermatozoa. 



The female cells, known as "oogonia," after a period of repeated 

 mitotic division also have a growth period with two maturation divisions 

 (Fig. ZA). UnUke the male, where all the spermatocytes finally develop 

 into spermatozoa, in the female maturation results in one functional egg 

 which carries all the nutrient material and three much smaller non- 

 functional cells known as "polar bodies." In some cases the second 

 polar body fails to divide, in which case but two polar bodies are formed. 



The chromosome behavior during one of the maturation divisions 

 differs from that of normal mitotic division in that the chromosomes of 

 each cell undergo a pairing which is known as "synapsis." It is not a 

 random pairing but is a union of homologous male and female chromo- 

 somes descended from parent chromosomes which came together at the 

 time the egg was fertilized. Chromosomes in many cases are sufficiently 

 distinctive morphologically so that homologous pairs are readily recog- 

 nized. Two of them, one derived from each parent, in the female egg 

 cell before synapsis are known as " a;-chromosomes " ; the others are known 

 as "autochromosomes." We shall now refer to the diagram (Fig. 3) 

 to illustrate what occurs during maturation of the germ cells of an animal 

 in which we shall assume that the male has five chromosomes and the 

 female six. The circle at Aa represents an egg before maturation with 

 six chromosomes; the circle Ba', a spermatocyte with five chromosomes. 

 At the first division of the egg nucleus reduction in number of chromo- 

 somes occurs (Ah), one of each pair going to the first polar body, the 

 others remaining in the egg. The second division is mitotic ; hence after 

 division at Ac there are still three chromosomes in the egg nucleus as in 

 the second polar body. In the spermatocyte (Ba') we are assuming that 

 there are five chromosomes, two pairs and a single x-chromosome, which 

 after reduction would give three chromosomes in one of the spermatocytes 

 of the second order and two in the other {Bh'). The second maturation 

 division (Be'), being mitotic, gives three chromosomes in one pair of 

 spermatids and two in the other pair, the same number being preserved 

 in the resulting spermatozoa (d). Should the egg (Ac) be fertihzed (at e) 

 by one of the sperms containing an a;-chromosome, a female animal will 

 develop. Should it be fertilized by a sperm lacking an a:-chromosome 

 (i.e., having but two chromosomes in our example), a male animal would 

 result. 



