CHA nike vil 
DEVELOPMENT 
1. EMBRYOLOGY 
Ovum.—The ovum of an insect, as of any other animal, 
is a single cell (Fig. 187), with a large nucleus (germinal 
Hie, 187: 
Sagittal section of egg 
of fly, Musca, in process 
of fertilization. c, cho- 
rion; d, dorsal; m, 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. 
vesicle), a large nucleolus, nutritive mat- 
ter, or yolk (deutoplasm), 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, 
however—the polar bodies—are minute 
and rudimentary. 
These phenomena ot ovogenesis are 
paralleled in the development of the sper- 
matozoa, or spermatogenesis ; for the pri- 
mary spermatocyte gives rise to two sec- 
ondary spermatocytes, and these to four 
spermatids, each of which forms a sper- 
matozoon. 
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 sim- 
ilar reduction occurs also during the de- 
velopment of the spermatozo6n, and when sperm-nucleus and 
146 
