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THE AMERICAN NATURALIST [Vol. LI 



ultimate oogonia are ready to enter upon the growth 

 period. A period of differentiation may or may not in- 

 tervene, according to the species, during which nurse 

 cells are formed. When the oocytes have reached their 

 full size they separate from the ovarian tubule, pass down 

 ■ the oviduct into the vagina and are deposited. Each egg 

 is surrounded by two membranes; a thin inner vitelline 

 membrane and a thicker, outer membrane, the chorion. 



6. The Complexity of Organization of the Insect Egg. 



(a) Comparison between Eggs of Insects amd Those of 

 Other ^mmaZs.— Insect eggs differ greatly from those 

 usually employed for the study of egg organization, since 

 they are, as a rule, laid in the air and not in the water, 

 and because cleavage is of the superficial type, cell walls 

 being absent until a comparatively late cleavage stage. 

 The eggs of chrysomelid beetles are particularly favor- 

 able for study, since they may be subjected to the most 

 violent experimental conditions without preventing their 

 development.^^ 



In insect eggs the character of the blastoderm cells 

 depends, as in holoblastic eggs, upon the kinds of proto- 

 plasm they contain, but all those phenomena connected 

 with the position of the cleavage spindle, which have been 

 so carefully studied in the eggs of mollusks, worms, 

 ascidians and other animals, can have no influence upon 

 the localization of different substances in various parts of 

 an insect egg, because in the latter the volume of the egg 

 is thousands of times greater than that of the cleavage 

 spindle. Furthermore in holoblastic eggs differentiated 

 substances are segregated in different cells during early 

 cleavage and are there isolated by cell walls, and to this 

 isolation is attributed in large part the progress of dif- 

 ferentiation ; but in the insect egg the different kinds of 

 cytoplasm are in direct continuity until hundreds of cleav- 

 age nuclei are present, and are not separated by cell walls 

 until the blastoderm is fully formed. 



i2Heg7ier, 1908, Biol. Bull., Vol. 16; 1909, Journ. Exp. Zool., Vol. 6; 

 1911, Biol. Bull., Vol. 20. 



