. CHAPTER III 

 DEVELOPMENT 



i. EMBRYOLOGY 



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

 cell (Fig. 190), with a large nucleus (germinal vesicle) -, Sil&rgenucleolus, 

 nutritive matter, or yolk (deutoplasm), contained in cytoplasm, and a 

 cell wall (mtelline 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 rudi- 

 mentary. 



These phenomena of ovogenesis are paralleled 

 in the development of the spermatozoa, or sperma- 

 togenesis; for the primary spermatocyte gives rise 

 to two secondary spermatocytes, and these to four 

 spermatids, 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 simi- 

 lar reduction occurs also during the develop- 

 ment of the spermatozoon, and when sperm-nucleus 

 and egg-nucleus unite, the resulting nucleus con- 

 tains 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 

 spermatozoa, previously stored in the seminal receptacle. Through 

 the micro pyle of the chorion one or more spermatozoa enter and a 



9 I2Q 



-pr 



FIG. 190. Sagittal 

 section of egg of fly, 

 Musca, in process of fer- 

 tilization, c, chorion; d, 

 dorsal; m, micropyle, 

 with gelatinous exuda- 

 tion; p, male and female 

 pronuclei, before union; 

 pb, polar bodies; pr, 

 peripheral protoplasm; 

 v, ventral; vt, vitelline 

 membrane; y, yolk. 

 After HEN KING and 

 BLOCHMANN. 



