CHAPTER II 

 A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



Before taking up other topics pertaining to the embryology of the 

 Insecta we beheve that a clearer conception of the various steps in their 

 natural sequence can be gained by first following through a brief outline 

 of the development of a typical insect from the time of maturation of the 

 egg nucleus to the emergence of the larva from the egg. Exceptional 

 types of development are not considered here but will be treated in the 

 chapters of Part II. 



This account does not apply to any specific insect but rather to a 

 generalized type that possesses characteristics common to many insects 

 in most particulars. The writers herein visualize an insect embryo that 

 possesses features that relate it to both generalized and specialized forms, 

 having an oval egg, a superficial germ band of moderate length forming 

 on the ventral side of the egg, with the germ cells making their appearance 

 before the formation of the germ layers, with bipolar development of 

 the mid-gut epithelium, with complete amnion and serosa, and with well- 

 developed coelomic sacs. 



The Egg (Fig. 4). — The oval egg, slightly flattened on the dorsal side, 

 is covered by a tough shell, or chorion (ch), beneath which is the thin 

 vitelline membrane. Its contents consist of a thin layer of protoplasm 

 (pr) at the surface (periplasm) and an internal protoplasmic reticulum (r) 

 in the meshes of which the food j^olk is held. The egg nucleus (nu) is at 

 or near the center. 



Entrance of Sperm. — Before an egg is deposited, one or more sperma- 

 tozoa may pass from the spermatheca of the female into the egg through 

 the micropyle, an opening or a group of openings in the shell in many 

 species located at the anterior end of the egg. 



Maturation. — About the time a sperm enters the micropyle, the female 

 nucleus migrates to the periphery of the egg where it undergoes two 

 maturation divisions, throwing off two or sometimes three polar bodies 

 (in case the first body divides again) (Fig. 5, ph). 



Fusion Nucleus. — The female nucleus, now with the reduced, or 

 haploid, number of chromosomes, migrates back toward the center of 

 the egg, uniting on the way with the nucleus of the sperm producing the 

 zygote, or fusion nucleus, in which the diploid number of chromosomes 

 has been restored. 



