EXPERIMENTAL EMBRYOLOGY 145 



The degree of determination existing in the egg at a given stage of 

 development is reflected in the ability of the egg to readjust itself toward 

 the production of a normal embryo after a defect is imposed upon it. 

 The more fixed the determination the less the power of regulation. 

 Insect eggs may accordingly be grouped into (1) indeterminate, (2) incom- 

 pletely determinate, and (3) determinate types. An indeterminate type of 

 egg is one in which the parts of the embryo are not predetermined at the 

 time of fertilization, and hence an egg with great regulative powers under 

 experimental conditions. A determinate type of egg is one in which the 

 parts are wholly predetermined before or during fertilization, and hence 

 an egg with Httle or no regulative power and given to mosaic formation 

 under experimental conditions. An incompletely determinate type of 

 egg is intermediate between these two. A graded series is found from 

 the highly indeterminate eggs in certain of the lower insect orders 

 through incompletely determinate eggs to the completely determinate 

 eggs of the higher Diptera. This indeterminate-determinate series will 

 be discussed in detail later. 



EXPERIMENTAL MATERIALS AND METHODS 



The forms experimented upon include Orthoptera, Odonata, Cole- 

 optera, Diptera, and Hymenoptera. Both aquatic and terrestrial types 

 have been used; study of the development of the latter is frequently 

 faciUtated by immersion in w^ater. Observations on living eggs are pos- 

 sible when they are naturally transparent or translucent and in some 

 cases after removal of the chorion. In experimental studies normal 

 eggs are always used as controls, and in some problems "experimental 

 controls" are also used (e.g., partially versus completely constricted 

 eggs). The experimental techniques that have been used may be sum- 

 marized as follows: 



1. Cauterization. — (a) Killing a small region or several regions with 

 a hot needle either to produce a minute scar or to eliminate certain 

 areas. (6) Light cauterization to produce localized contractions of the 

 yolk system, (c) Unilateral heating with a microcauterizer to produce 

 minute splits by unequal expansion of the egg materials. 



2. Irradiation with ultraviolet light to (a) kill selected nuclei, (6) kill 

 certain areas of the egg, (c) cause temporary changes in the cytoplasm, 

 (d) produce a minute scar, or (e) observe the effect of in toto irradiation 

 of oriented eggs. 



3. Puncturing the egg with a cold needle, either to allow part of the 

 egg contents to flow out or to injure, divide, or alter the germ band. 



4. Producing yolk fissures by bending the egg. 



5. Constricting the egg (completely or incompletely) at various points 

 with a fine hair. 



