CHAPTER III 



DEVELOPMENT 



i. EMBRYOLOGY 



Ovum. The ovum of an insect, as of any other animal, is a single 

 cell (Fig. 188), with a large nucleus (germinal vesicle*), & large nucleolus, 

 nutritive matter, or yolk (deuto plasm), contained in the cytoplasm, and a 



cell wall (vitelline membrane} secreted by the ovum; 

 the egg-shell, or chorion, is secreted around the 

 ovum by surrounding ovarian cells. 



Maturation. As a preparation for fertilization 

 the germinal vesicle divides twice, forming two polar 

 bodies, and as the first of these bodies may itself 

 divide, there result four cells; three of these, how- 

 ever the polar bodies are minute and rudimen- 

 tary. 



These phenomena of orogenesis are paralleled in 

 the development of the spermatozoa, or spermato- 

 genesis; for the primary spcrmatocyte gives rise to 

 two secondary spcrmatocytes, and these to four sper- 

 matids, each of which forms a spermatozoon. 



By means of this maturation process the number 

 of chromosomes in the egg-nucleus is reduced to half 

 the number normal for somatic cells (body cells as 

 distinguished from germ cells) . A similar reduction 

 occurs also during the development of the sperma- 

 tozoon, and when sperm-nucleus and egg-nucleus 

 unite, the resulting nucleus contains the normal 

 number of chromosomes. The meaning of these 

 reduction phenomena highly important from the 

 standpoint of heredity is a much debated subject. 

 Fertilization. As the eggs pass through the 

 vagina, they are capable of being fertilized by sper- 

 matozoa, previously stored in the seminal recep- 

 tacle. Through the micro pyle of the chorion one or more spermatozoa 

 enter and a sperm nucleus unites with the egg-nucleus to form what is 



nS 



FIG. 188 Sagittal 

 section of egg of fly, 

 Aliisca, in process of 

 fertilization. c, cho- 

 rion; d, dorsal; ;, mi- 

 cropyle, with gelatinous 

 exudation; p, male and 

 female pronuclei, before 

 union; pb, polar bodies; 

 pr, peripheral proto- 

 plasm; v, ventral; vt, 

 vitelline membrane; y, 

 yolk. After HENKING 

 and BLOCHMANN. 



