24 



CHAPTER 2 



figure 2-7. Egg, mature larva, and pupa of Drosophila 

 melanogaster. Each unit of scale equals 1 mm. (Cour- 

 tesy of E. R. Balboni. ) 



The adult male is diploid and has a pair 

 of testes in which spermatogonia are pro- 

 duced by mitosis. When one of these sper- 

 matogonia! cells enters meiosis it is called 

 a primary spermatocyte. The first meiotic 

 division produces two secondary spermato- 

 cytes; the second meiotic division, four hap- 

 loid spermatids. Each spermatid differen- 

 tiates without further division into a sper- 

 matozoan, or sperm cell. Note that for each 

 diploid primary spermatocyte entering meio- 

 sis. four functional haploid sperm are pro- 

 duced at the completion of spermatogenesis. 

 This is also true of males of many higher 

 animals, including the frog, mouse, and man. 

 Sperm are stored in the Drosophila male 

 until they are ejaculated into the vagina of 

 the female, where they proceed to swim into 

 the female's sperm storage organs (a pair of 

 spermathecae and a coiled ventral recep- 

 tacle). 



The adult female has a pair of ovaries 

 each of which is composed of a series of 

 egg tubes, or ovarioles. At one end of the 

 ovariole are diploid oogonia. By four syn- 

 chronous mitotic divisions each oogonium 

 produces a nest of 16 cells, one of which 

 enters meiosis as a primary oocyte while 

 those remaining serve as nurse cells for the 

 maturing oocyte. As the oocyte grows it 

 passes down the ovariole, into the oviduct 

 and then the uterus. At the time it reaches 



the uterus, the egg (Figure 2-7) is usually 

 no further advanced than metaphase of the 

 first meiotic division. The sperm held in 

 the female are released to fertilize the egg 

 in the uterus, after which the first meiotic 

 division continues. The two secondary 

 oocyte nuclei produce four haploid nuclei, 

 three of which are polar nuclei and degen- 

 erate, the remaining one serving as the hap- 

 loid egg nucleus. Note here, as in females 

 of the frog, mouse, and man, that a single 

 primary oocyte produces only one mature 

 haploid egg at the completion of oogenesis. 

 (In man, also, meiosis is completed after 

 fertilization, the egg remaining at metaphase 

 II until sperm entry.) Since the female 

 Drosophila stores hundreds of sperm and 

 uses them sparingly (only one sperm usu- 

 ally enters the egg), a single mating can 

 yield hundreds of progeny. 



At about 25° C embryonic development 

 proceeds for about a day, when the larva 

 hatches from the egg. After four more days 

 and two moltings, the mature larva becomes 

 a pupa (Figure 2-7) . After about four days 

 the young adult, or imago, ecloses (hatches) 

 from the pupa case. Thus, the Drosophila 

 undergoes a complete metamorphosis during 

 its life cycle. Although mating usually oc- 

 curs during the first 24 hours of adult life, 

 oviposition usually starts during the second 

 day, so that the generation time is about 10 



