CHAPTER III 



DEVELOPMENT 



i. EMBRYOLOGY 



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

 is a single cell (Fig. 187), with a large nucleus (germinal 

 vesicle) , a large nucleolus, nutritive mat- 

 ter, or yolk (dent o 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, 

 however the polar bodies are minute 

 and rudimentary. 



These phenomena of orogenesis are 

 paralleled in the development of the sper- 

 matozoa, or spermato genesis; 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 spermatozoon, and when sperm-nucleus and 



146 



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. 



