10 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Division of the Nucleus. — On reaching the egg center, or before it 

 has been reached, the zygote nucleus undergoes mitotic division, the 

 daughter nuclei (cleavage nuclei) migrating toward the egg periphery. 

 In their outward migration the nuclei acquire a cytoplasmic envelope 

 derived from the protoplasmic reticulum, thus becoming amoeba-like 

 cleavage cells (Fig. 6). 



Germ Cells, or Sex Cells, and Germ-track Determinant. — Before the 

 primary epithelium is complete, some of the cleavage cells migrating 



-OS OS—/ OS 



Fig. 4. Fig. 5. Fig. 6. 



Fig. 4. — Insect egg. Fig. 5. — Maturation. Fig. 6. — Cleavage, {cc) Cleavage 

 nucleus, {ch) Chorion, {mi) Micropyle. {nu) Nucleus of egg. (os) Oosome. {ph) 

 Polar bodies, {pr) Periplasm, (r) Protoplasmic reticulum, (sp) Sperm. The chorion 

 has been omitted in Figs. 5 and 6. 



toward the posterior pole of the egg pass on their way through a proto- 

 plasmic substance variously designated by writers as "oosome," "germ- 

 track determinant," "Keimbahn determinant," or "polar plasm" 

 (Fig. 6, os). The cells in passing through this oosome cause it to break 

 up the fragments accompanying them as they form a clump at the pos- 

 terior pole of the egg (Fig. 7). This mass of cells are the germ cells {gc), 

 also called "polar globules" or "pole cells" by earlier writers. 



The Primary Epithelium, or So-called " Blastoderm." — The cleavage 

 cells now penetrate the periplasm (Fig. 8), undergo one or more mitotic 

 divisions, and then acquire distinct cell walls. The yolk is thus now 



